After a great struggle, I finally got sound working somewhat on my system. I'm using ALSA (kernel 2.6.10, debian), in KDE.

The sounds for system functions (like opening a program) play fine, but I can't seem to get anything to play using aplay.

I think for some reason it thinks I have 2 sound devices, and the first one is non-existant which is why it's not working. When I go to XMSS's config for ALSA, it lists 3 audio devices:
Default PCM device (default)
Ensoniq AudioPCI : ES1370 DAC2/ADC (hw:0,0)
Ensoniq AudioPCI : ES1370 DAC1 (hw:0,1)

if I choose default, it doesn't work (is defaulting to the "hw:0,0" I think). If I choose the "hw:0,0" it also doesn't work. However if I choose "hw:0,1" it works great.

So my question is, how can I change the "default" to be the other device, so that aplay works?

Try running alsaconf, and see what that returns. I suspect that you have multiple device files associated with one sound card; this may be normal for your card, it is for mine.

I tried it, and it went through fine... only listed 1 card there.

Still 2 devices + default listed in xmss though, and things like:
aplay test.wav
don't work.