I had a Clié TH55, and it's mp3 software was friendlier, but I think the Zaurus makes up for it with its large capacity HD. The Zaurus software is not bad, but i just think the clié was more user friendly.
You have to keep the Zaurus open to change volume(you can use the scroll button to switch songs though), there is no "hold" function to prevent accidental button pressing, you cannot change the sound settings (equalizer), limit the sound volume to prevent hearing loss. These are not "vital" functions, but they're nice to have.
Sound quality-wise Cliés were known for their superb sound, I think the SL-C3K is pretty good.
I do not know the specs for the sound, but i can tell you it's mostly about the headphones. I tried the Zaurus with earbuds and i thought it sucked big time, I tried big studio type headphones, and i thought the sound was pretty good. You need the bass function from the clie with the earbud kind so that you have some deep sounds. when you have the big headphones, then you really don't need the bass because it distorts music too much. Then again it's also a matter of what you like to listen to and how you like your sound settings. I listen to classic music, acid jazz and trance music (i wonder how many people out there will listen to such a wide range of musical choice).
The notes are clear, but if you like the way it sounds really depends on your taste and the headphones you use.
-Alberto