I know of a few minor differences. For NSF,
nonlinear sound mixing for isn't implemented in the public release, since it's almost unnoticeable (I can send support if this is really desired). I also don't know the exact mix levels of the external sound chips (VRC6, Namco 106, FME-7), so these might be slightly off. The other aspects of sound should be very nearly perfect, though I still haven't written a complete set of
verification ROMs for the NES APU.
For SPC, I've addressed all issues I've encountered so far. I know of a few differences in the DSP core that I haven't bothered to add (clamp before fourth Gaussian lookup, for example), since I haven't set up a good test framework. It shouldn't be hard to compare my DSP core to the latest docs to work out these, if anyone wants to. If they have an effect on sound, I haven't noticed it. I've done comparisons with sound track recordings from my SNES and found no audible difference.
I'm still actively working on my
Game_Music_Emu sound engine (which AO uses) and welcome reports of possible inaccuracies. First check my list of
buggy game music for things I've already verified to sound "wrong" on the actual system.