audio_gcd
np = bytes / audio_gcd(p->bps * p->channels, bytes);
nr = bytes / audio_gcd(r->bps * r->channels, bytes);
return nr * np / audio_gcd(nr, np);
np = blk_mult / audio_gcd(sc->pchan * sc->bps, blk_mult);
nr = blk_mult / audio_gcd(sc->rchan * sc->bps, blk_mult);
mult = nr * np / audio_gcd(nr, np);