for some reason my mp3 sound quality is awful in linux. .ogg's sound fine, cd's sound fine, wav's sound ok.

All formats in general are slightly worse quality than one a windows machine, but for some reason mp3's sound horrendous

any ideas?

okay, need more information.
what bitrate are thye mp3s, what program are you playing them with?
waht soundcard do you have and what drivers is it using?

A long shot:

You are using xmms for playback and you have "Preamp" in the equalizer settings at max. Turn that down a little...

Martin

I had the same problem and it was driving me to hell. In slackware 9.1 I couldn't listen to mp3's because the quality was just hellishly scrachy, like the volume was blared when it wasn't. In other games the quality was great. If this sounds like your problem here is what I changed.
1. Go to SystemSettings>Control Center>Sound/Multimedia>Sound System
2. Go to the Sound I/O tab
3. Change sound quality from Autodetect to 16bits(high)
4. I also change the Audio Buffer Size (response time) from the middle all the way to the left.(8 millieseconds)
5. Lastly I checked enable full duplex operation.(I don't know how much this effected it if any at all.

Hope this helps!