Based on PocketCGD Github
Clone this project inside the sf2000_multicore/cores/ folder.
Command:
git clone https://github.com/GrGadam/froggyMP3.git --recurse-submodules
In the sf2000_multicore folder use the following command:
make CONSOLE=froggymp3 CORE=cores/froggyMP3
After this, the built core should be under sf2000_multicore/sdcard/cores/froggyMP3
First time, you can move the whole folder on to your sf2000 (which must have multicore installed) in the cores folder (like: /cores/froggymp3)
After that, you need to copy your .mp3 files to your sf2000 CORES/froggymp3/. (If the folder doesn't exist first, you can just create a new folder and name it froggymp3).