It might be that the rate dmix is using isn't the same that your
card supports natively. Of course, I haven't looked at the Rawhide
updates yet so I could be wrong...
I just changed some settings in xmms and it appears that this might be a
Gnome, of course, uses esound for it's stuff, and I had though I was
using the alsa plugin for xmms, but it turns out I was using the esound
plugin. After switching to also, xmms now plays music very nicely.
It should be noted that I'm using the current rawhide packages as I
downgraded them to try this (and got no sound from esound at all).