ok i know a game that you should look a bit if you have time: Top Gear Rally (and may be Stunt Racer and World driver championship)
it's a Boss Games studio. All Boss Games studio have problem with sound because i think that they are using a different way either to use sound or because they have developed their own audio system .
I think also that timings are causing a lot of problems in those games and at least with Top gear Rally the sound is nearly correct with your Audio fix.
But the sound in game is very strange: i think their is two main channels in this game: one for music, one for the car and so on sounds. Playing with timings help a lot to get better sound (in fact having sound) (check PM to understand what i mean by this) but with the RSP of PJ64 1.5 (or 1.6 as PJ 1.6 is out) the music channel seems to messing up.
it's as if the two channels didn't succeed to mix them up correctly or something like that (i'm unsure of that of course .... )
well that is just empiric test but may be it would be nice to see that
Thanks
BTW here a list of the Boss Game studio:
Stunt Racer 64, Top Gear Rally, Twisted Edge Extreme Snowboarding, World Driver Championship.
As you can see all those have major problems with audio or core/audio problem.
Twisted Edge Extreme Snowboarding: no sound
Top Gear Rally: only working with Audiofix edition and sound is not very good
Stunt Racer 64 and World Driver Championship: doesn't pass the first screen whatever i tried : i think it's a core/audio problem(to have gfx use Rice video plugin)
PS: with Audiofix 2 i think it' s abit better (??)