diff --git a/packages/core/src/compiler/timingCompiler.test.ts b/packages/core/src/compiler/timingCompiler.test.ts
index 41bbb4773..035b936e1 100644
--- a/packages/core/src/compiler/timingCompiler.test.ts
+++ b/packages/core/src/compiler/timingCompiler.test.ts
@@ -11,6 +11,16 @@ describe("compileTimingAttrs", () => {
     const html = '<video id="v1" src="a.mp4" data-start="2" data-duration="5">';
     const { html: compiled, unresolved } = compileTimingAttrs(html);
 
+    expect(compiled).toContain('data-end="7"');
+    expect(compiled).not.toContain("data-has-audio");
+    expect(unresolved).toHaveLength(0);
+  });
+
+  it("preserves explicit video audio declarations", () => {
+    const html =
+      '<video id="v1" src="a.mp4" data-start="2" data-duration="5" data-has-audio="true">';
+    const { html: compiled, unresolved } = compileTimingAttrs(html);
+
     expect(compiled).toContain('data-end="7"');
     expect(compiled).toContain('data-has-audio="true"');
     expect(unresolved).toHaveLength(0);
@@ -40,7 +50,7 @@ describe("compileTimingAttrs", () => {
     const { html: compiled, unresolved } = compileTimingAttrs(html);
 
     expect(compiled).toContain('id="hf-video-0"');
-    expect(compiled).toContain('data-has-audio="true"');
+    expect(compiled).not.toContain("data-has-audio");
     expect(unresolved).toHaveLength(1);
     expect(unresolved[0].id).toBe("hf-video-0");
     expect(unresolved[0].tagName).toBe("video");
diff --git a/packages/core/src/compiler/timingCompiler.ts b/packages/core/src/compiler/timingCompiler.ts
index a9fde0b60..51a4574da 100644
--- a/packages/core/src/compiler/timingCompiler.ts
+++ b/packages/core/src/compiler/timingCompiler.ts
@@ -7,7 +7,6 @@
  * Guarantees every timed element gets:
  * - id on media elements when missing
  * - data-end (computed from data-start + data-duration when possible)
- * - data-has-audio="true" on <video> elements
  *
  * For elements without data-duration (e.g. videos relying on source duration),
  * this compiler identifies them as "unresolved" so the caller can provide
@@ -101,11 +100,6 @@ function compileTag(
     }
   }
 
-  // 2. Add data-has-audio="true" to <video> elements
-  if (isVideo && !hasAttr(result, "data-has-audio")) {
-    result = injectAttr(result, "data-has-audio", "true");
-  }
-
   return { tag: result, unresolved };
 }
 
@@ -113,7 +107,7 @@ function compileTag(
  * Compile timing attributes in HTML.
  *
  * Phase 1 (static): Adds data-end where data-duration exists,
- * adds data-has-audio on videos.
+ * preserves explicit video audio declarations.
  *
  * Returns the compiled HTML and a list of elements that could not be
  * resolved statically (missing data-duration). The caller should resolve
diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index d2dff7f6f..b4e9d3aae 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -97,6 +97,7 @@ export {
   ENCODER_PRESETS,
   getEncoderPreset,
   type GpuEncoder,
+  type MuxOptions,
 } from "./services/chunkEncoder.js";
 export type { EncoderOptions, EncodeResult, MuxResult } from "./services/chunkEncoder.types.js";
 
diff --git a/packages/engine/src/services/chunkEncoder.test.ts b/packages/engine/src/services/chunkEncoder.test.ts
index 6a2fe93c5..287a4c02b 100644
--- a/packages/engine/src/services/chunkEncoder.test.ts
+++ b/packages/engine/src/services/chunkEncoder.test.ts
@@ -1,5 +1,23 @@
 import { describe, it, expect, vi } from "vitest";
-import { ENCODER_PRESETS, getEncoderPreset, buildEncoderArgs } from "./chunkEncoder.js";
+import { runFfmpeg } from "../utils/runFfmpeg.js";
+import {
+  ENCODER_PRESETS,
+  getEncoderPreset,
+  buildEncoderArgs,
+  muxVideoWithAudio,
+} from "./chunkEncoder.js";
+
+vi.mock("../utils/runFfmpeg.js", () => ({
+  runFfmpeg: vi.fn(async () => ({
+    success: true,
+    exitCode: 0,
+    stderr: "",
+    durationMs: 12,
+  })),
+  formatFfmpegError: vi.fn((exitCode: number | null, stderr: string) =>
+    exitCode === null ? stderr : `FFmpeg exited with code ${exitCode}`,
+  ),
+}));
 
 describe("ENCODER_PRESETS", () => {
   it("has draft, standard, and high presets", () => {
@@ -78,6 +96,46 @@ describe("getEncoderPreset", () => {
   });
 });
 
+describe("muxVideoWithAudio", () => {
+  it("clamps mux output to the encoded video frame duration when provided", async () => {
+    const mockedRunFfmpeg = vi.mocked(runFfmpeg);
+    mockedRunFfmpeg.mockClear();
+
+    await muxVideoWithAudio("video-only.mp4", "audio.aac", "output.mp4", undefined, {
+      ffmpegProcessTimeout: 1234,
+      durationSeconds: 500 / 30,
+    });
+
+    expect(mockedRunFfmpeg).toHaveBeenCalledTimes(1);
+    const [args, options] = mockedRunFfmpeg.mock.calls[0] ?? [];
+    expect(args).toEqual(
+      expect.arrayContaining([
+        "-map",
+        "0:v:0",
+        "-map",
+        "1:a:0",
+        "-c:v",
+        "copy",
+        "-t",
+        String(500 / 30),
+      ]),
+    );
+    expect(args).not.toContain("-shortest");
+    expect(options).toEqual(expect.objectContaining({ timeout: 1234 }));
+  });
+
+  it("falls back to shortest-stream muxing when no target duration is known", async () => {
+    const mockedRunFfmpeg = vi.mocked(runFfmpeg);
+    mockedRunFfmpeg.mockClear();
+
+    await muxVideoWithAudio("video-only.mp4", "audio.aac", "output.mp4");
+
+    const [args] = mockedRunFfmpeg.mock.calls[0] ?? [];
+    expect(args).toContain("-shortest");
+    expect(args).not.toContain("-t");
+  });
+});
+
 describe("buildEncoderArgs anti-banding", () => {
   const baseOptions = { fps: 30, width: 1920, height: 1080 };
 
diff --git a/packages/engine/src/services/chunkEncoder.ts b/packages/engine/src/services/chunkEncoder.ts
index 594cda66d..ca3415875 100644
--- a/packages/engine/src/services/chunkEncoder.ts
+++ b/packages/engine/src/services/chunkEncoder.ts
@@ -243,6 +243,10 @@ export function buildEncoderArgs(
   return args;
 }
 
+export interface MuxOptions extends Partial<Pick<EngineConfig, "ffmpegProcessTimeout">> {
+  durationSeconds?: number;
+}
+
 export async function encodeFramesFromDir(
   framesDir: string,
   framePattern: string,
@@ -494,14 +498,14 @@ export async function muxVideoWithAudio(
   audioPath: string,
   outputPath: string,
   signal?: AbortSignal,
-  config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout">>,
+  config?: MuxOptions,
 ): Promise<MuxResult> {
   const outputDir = dirname(outputPath);
   if (!existsSync(outputDir)) mkdirSync(outputDir, { recursive: true });
 
   const isWebm = outputPath.endsWith(".webm");
   const isMov = outputPath.endsWith(".mov");
-  const args = ["-i", videoPath, "-i", audioPath, "-c:v", "copy"];
+  const args = ["-i", videoPath, "-i", audioPath, "-map", "0:v:0", "-map", "1:a:0", "-c:v", "copy"];
 
   if (isWebm) {
     args.push("-c:a", "libopus", "-b:a", "128k");
@@ -510,7 +514,14 @@ export async function muxVideoWithAudio(
   } else {
     args.push("-c:a", "aac", "-b:a", "192k", "-movflags", "+faststart");
   }
-  args.push("-shortest", "-y", outputPath);
+
+  const durationSeconds = config?.durationSeconds;
+  if (durationSeconds !== undefined && Number.isFinite(durationSeconds) && durationSeconds > 0) {
+    args.push("-t", String(durationSeconds));
+  } else {
+    args.push("-shortest");
+  }
+  args.push("-y", outputPath);
 
   const processTimeout = config?.ffmpegProcessTimeout ?? DEFAULT_CONFIG.ffmpegProcessTimeout;
   const result = await runFfmpeg(args, { signal, timeout: processTimeout });
diff --git a/packages/producer/src/services/chunkEncoder.ts b/packages/producer/src/services/chunkEncoder.ts
index 87ce12a4b..ab013e1ae 100644
--- a/packages/producer/src/services/chunkEncoder.ts
+++ b/packages/producer/src/services/chunkEncoder.ts
@@ -12,5 +12,6 @@ export {
   type EncoderOptions,
   type EncodeResult,
   type MuxResult,
+  type MuxOptions,
   type GpuEncoder,
 } from "@hyperframes/engine";
diff --git a/packages/producer/src/services/compilationTester.ts b/packages/producer/src/services/compilationTester.ts
index 62f4d8310..270335d69 100644
--- a/packages/producer/src/services/compilationTester.ts
+++ b/packages/producer/src/services/compilationTester.ts
@@ -167,11 +167,6 @@ function validateElementTiming(element: CompiledElement, label: string): string[
     }
   }
 
-  // Video-specific: require data-has-audio
-  if (element.tagName === "video" && element.dataHasAudio === undefined) {
-    errors.push(`${label} [${element.id}]: missing data-has-audio attribute`);
-  }
-
   return errors;
 }
 
diff --git a/packages/producer/src/services/htmlCompiler.test.ts b/packages/producer/src/services/htmlCompiler.test.ts
index d7daa58c1..7a5c1548f 100644
--- a/packages/producer/src/services/htmlCompiler.test.ts
+++ b/packages/producer/src/services/htmlCompiler.test.ts
@@ -490,10 +490,51 @@ describe("detectShaderTransitionUsage", () => {
   });
 });
 
+describe("video audio declarations", () => {
+  it("does not synthesize audible video audio for muted visual tracks", async () => {
+    const projectDir = mkdtempSync(join(tmpdir(), "hf-muted-video-audio-"));
+    writeFileSync(
+      join(projectDir, "index.html"),
+      `<!doctype html>
+<html>
+  <body>
+    <div data-composition-id="main" data-width="640" data-height="360" data-duration="2">
+      <video
+        id="visual"
+        src="clip.mp4"
+        muted
+        playsinline
+        data-start="0"
+        data-duration="2"
+      ></video>
+      <audio id="mix" src="mix.wav" data-start="0" data-duration="2"></audio>
+    </div>
+  </body>
+</html>`,
+    );
+
+    const compiled = await compileForRender(projectDir, join(projectDir, "index.html"), projectDir);
+
+    expect(compiled.videos).toEqual([
+      expect.objectContaining({
+        id: "visual",
+        hasAudio: false,
+      }),
+    ]);
+    expect(compiled.audios).toEqual([
+      expect.objectContaining({
+        id: "mix",
+        type: "audio",
+      }),
+    ]);
+    expect(compiled.html).not.toContain('data-has-audio="true"');
+  });
+});
+
 describe("template-wrapped sub-composition media offsets", () => {
   function writeTemplateWrappedProject(
     hostAttrs: string,
-    mediaAttrs: string = 'data-start="0" data-duration="4"',
+    mediaAttrs: string = 'data-start="0" data-duration="4" data-has-audio="true"',
     extraMediaMarkup: string = "",
   ): {
     projectDir: string;
@@ -613,7 +654,7 @@ describe("template-wrapped sub-composition media offsets", () => {
   it("offsets scene-local media in compositions that start much later on the timeline", async () => {
     const { projectDir, indexPath } = writeTemplateWrappedProject(
       'data-start="20" data-duration="6" data-width="640" data-height="360"',
-      'data-start="1.5" data-duration="4"',
+      'data-start="1.5" data-duration="4" data-has-audio="true"',
     );
 
     const compiled = await compileForRender(projectDir, indexPath, projectDir);
diff --git a/packages/producer/src/services/renderOrchestrator.ts b/packages/producer/src/services/renderOrchestrator.ts
index c8a7c3153..365fabd80 100644
--- a/packages/producer/src/services/renderOrchestrator.ts
+++ b/packages/producer/src/services/renderOrchestrator.ts
@@ -2250,24 +2250,24 @@ export async function executeRenderJob(
       videoMetadataHints = collectVideoMetadataHints(extractionResult.extracted);
       perfStages.videoExtractMs = Date.now() - stage2Start;
 
-      // Auto-detect audio from video files via ffprobe metadata
+      // Only explicitly audible videos contribute audio. Muted visual video
+      // tracks often have embedded scratch/source audio while the composition
+      // uses a separate <audio> element for the final mix.
       const existingAudioSrcs = new Set(composition.audios.map((a) => a.src));
       for (const ext of extractionResult.extracted) {
-        if (ext.metadata.hasAudio) {
-          const video = composition.videos.find((v) => v.id === ext.videoId);
-          if (video && !existingAudioSrcs.has(video.src)) {
-            composition.audios.push({
-              id: `${video.id}-audio`,
-              src: video.src,
-              start: video.start,
-              end: video.end,
-              mediaStart: video.mediaStart,
-              layer: 0,
-              volume: 1.0,
-              type: "video",
-            });
-            existingAudioSrcs.add(video.src);
-          }
+        const video = composition.videos.find((v) => v.id === ext.videoId);
+        if (video?.hasAudio && ext.metadata.hasAudio && !existingAudioSrcs.has(video.src)) {
+          composition.audios.push({
+            id: `${video.id}-audio`,
+            src: video.src,
+            start: video.start,
+            end: video.end,
+            mediaStart: video.mediaStart,
+            layer: 0,
+            volume: 1.0,
+            type: "video",
+          });
+          existingAudioSrcs.add(video.src);
         }
       }
     } else {
@@ -3667,6 +3667,10 @@ export async function executeRenderJob(
           audioOutputPath,
           outputPath,
           abortSignal,
+          {
+            ffmpegProcessTimeout: cfg.ffmpegProcessTimeout,
+            durationSeconds: totalFrames / job.config.fps,
+          },
         );
         assertNotAborted();
         if (!muxResult.success) {
diff --git a/packages/producer/src/services/timingCompiler.ts b/packages/producer/src/services/timingCompiler.ts
index b08720360..136f129a5 100644
--- a/packages/producer/src/services/timingCompiler.ts
+++ b/packages/producer/src/services/timingCompiler.ts
@@ -73,10 +73,6 @@ function compileTag(
     }
   }
 
-  if (isVideo && !hasAttr(result, "data-has-audio")) {
-    result = injectAttr(result, "data-has-audio", "true");
-  }
-
   return { tag: result, unresolved };
 }
 
diff --git a/packages/studio/vite.config.ts b/packages/studio/vite.config.ts
index 6d99b3a36..32d69a69e 100644
--- a/packages/studio/vite.config.ts
+++ b/packages/studio/vite.config.ts
@@ -53,19 +53,21 @@ const THUMBNAIL_CACHE_VERSION = "v2";
 function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAdapter {
   // Lazy-load the bundler via Vite's SSR module loader
   let _bundler: ((dir: string) => Promise<string>) | null = null;
-  let _producerModulePromise: Promise<{
-    createRenderJob: (config: {
-      fps: 24 | 30 | 60;
-      quality: "draft" | "standard" | "high";
-      format: string;
-    }) => unknown;
-    executeRenderJob: (
-      job: unknown,
-      projectDir: string,
-      outputPath: string,
-      onProgress?: (job: { progress: number; currentStage?: string }) => void,
-    ) => Promise<void>;
-  }> | null = null;
+  let _producerModuleLoader:
+    | (() => Promise<{
+        createRenderJob: (config: {
+          fps: 24 | 30 | 60;
+          quality: "draft" | "standard" | "high";
+          format: string;
+        }) => unknown;
+        executeRenderJob: (
+          job: unknown,
+          projectDir: string,
+          outputPath: string,
+          onProgress?: (job: { progress: number; currentStage?: string }) => void,
+        ) => Promise<void>;
+      }>)
+    | null = null;
   const getBundler = async () => {
     if (!_bundler) {
       try {
@@ -80,8 +82,8 @@ function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAda
   };
 
   const getProducerModule = async () => {
-    if (!_producerModulePromise) {
-      _producerModulePromise = createRetryingModuleLoader(async () => {
+    if (!_producerModuleLoader) {
+      _producerModuleLoader = createRetryingModuleLoader(async () => {
         const { built } = ensureProducerDist({
           studioDir: __dirname,
           env: process.env,
@@ -93,9 +95,9 @@ function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAda
         }
         const producerPkg = "@hyperframes/producer";
         return await import(/* @vite-ignore */ producerPkg);
-      })();
+      });
     }
-    return _producerModulePromise();
+    return _producerModuleLoader();
   };
 
   return {