It seems the copy of etch I received won't be working without an internet connection, so I installed the copy of slackware 9.1 I had burried away... so all the software is pretty old.

Well, initially, I was getting nothing but static, so I added the dxs_support=2 parameter, 3 also worked but not as often as 2, 2 still cuts out sometimes, just less frequently. However, when playing music in xmms, it NEVER messes up. Sounds everywhere else still pop out, usually I have to pause/stop and restart the file until it just works, but xmms never has problems. It's not perfect sound quality, but the sounds are coherent. What would be causing it to work consistently here? I guess xmms has it's own module library? How could I make all sound perform the same way?

On-board sound is crap. Either get a sound card or use ALSA to convert all sounds set to a certain sampling rate. If you can get access to the internet, download the latest ALSA software.