A question about Sega Master System hacking

[Cris1997XX]

I've recently heard a great FM+PSG remix of Ken's theme from Street Fighter 2, and I wanted to add it to the 1997 Master System port. Where can I find code related to the audio side of things? I assume I need to rewrite some of it to implement compatibility with the FM add-on, but also be careful about playing on the Mark III (PSG is completely turned off for technical reasons)