System 7 (and OS 8) natively boots on the Mac mini G4!

[eharmon]

Depends on what code's being loaded -- if it's all PPC code, then that's not going to work on a 68k OS. If that can be tweaked to load into the ROM instead of into System memory, then it'll load before the 68k OS, and so PPC should work.

All that said, my OS 9 install works just fine with audio already? I just have issues with the CHRP-enabled OSes. There are two different ROM hacks enabling OS 9 on the Mini (before the CHRP one) -- one supports sound and the other doesn't.

This is adding support for onboard sound -- built-in speaker, and headphones at full volume.

USB sound and (quiet) headphone output has worked for awhile.

Which aligns with the problem as described: the amplifier is being turned off and needs to be re-enabled. Meaning OS 9 has been playing sound, it's just not amplified enough for the speaker to work.