Hi Dan,
In addition to what Andre says in his message (below), it wouldn't hurt to add a PPC assembly version of the FPM fixed-point multiply routines. Probably you're using the FPM_APPROX version now, so you could improve sound quality and possibly performance as well by writing a PPC version. See libmad/fixed.h for details.
BTW Dan, I can't tell if you're subscribed to mad-dev (under another address perhaps.) Let me know either way -- I assumed you were, but if not you should probably subscribe. If you are, you might want to change your subscription to match the address you send from so I don't have to manually approve your messages.
Cheers, -rob