A lot of games use the Z80 for FM and PSG access and let the 68K handle the DAC port (which is part of the YM2612, technically.) Some games may also drive the PSG with the 68K, IIRC.
If you can confirm that you're not hearing anything but DAC, then it's definitely an issue with the Z80 not running or communicating properly.
> i am currently working on the Genesis Plus Port for Gamecube as it appears some
> games doesn't have sound working properly (only fx sound, no music): Castle of
> Illusion and Quackshot for example, but also Thunderforce 4, Wonderboy (the
> shooting game) and Undead Line
> I was wondering what could be the possible cause of this:
> - unaccurate timing between Z80 and 68000 (bus request mechanism)
> - emulation bug (but the majority of games have music working just fine)
> - YM2612 or z80 or 68000 core bug
> If someone with good experience with the Genesis could help me finding where the
> problem could come from, i would appreciate a lot ;-)