This is a rewrite from scratch of my old MSU-1 music patch. It should be easier to use and have much cleaner code. Let me know if there's any bugs or issues with it. Features automatic SPC fallback in case of no MSU-1 or missing PCMs.
Get a fresh AMK and replace asm/main.asm, and asm/SNES/patch.asm with the ones in the folder. Place msu.asm in asm/SNES/.
Put an empty file named <romname>.msu in the same folder as your ROM (This is the MSU data. You can use it if you want).
Place PCM files named <romname>-<tracknum>.pcm in the same folder as your ROM. (You will need wav2msu. Also, global SMW tracks in PCM format are availablehere. See the old tutorial for more detailed instructions.)
EDIT: I have uploaded a new version. This one should properly handle P-switches and stars and fix the other bug Brutalpode mentioned.