I doubt I'll be adding support for any more formats, especially not those
from later-generation systems. I've covered most of the systems I grew up
with that had significant music. My main interests don't center around the
quantity of music formats, more on experimenting with ways of emulating
them. Plus, Game Boy Advance has crap sound quality. :)
I've made the full source code available in case anyone wants to work with
me or independently on a bigger game music player for Mac OS 9. I doubt
that many people develop for it anymore, though.
You might be able to run Mac OS X on your machine. I'm using a PowerMac
with an 8500/180 motherboard made in 1995 with a Sonnet 400MHz G3 processor
card with a 200MHz 1MB L2 cache and can run Mac OS X 10.3.9 decently (with
the help of XPostFacto), including the latest Audio Overload release
(which, by the way, now uses my NSF, GBS, and SPC sound engines from Game
Music Box).