I have a microsoft digital sound system 80 (sold in late 90s, i think
- picked mine up used just recently) and dell 4600 desktop i bought
last year. I can get the small desktop speakers to work, but not the
subwoofer. i have no documentation for the sound system, and can't
figure out how to troubleshoot. I'd like at least documentation for
speakers and hopefully help figuring out how to troubleshoot problem
as it may also be my sound card or something.

Here's something to try:

"Test the performance of the subwoofer. If the subwoofer is not
working properly, press and hold down the subwoofer button on the
satellite speaker."

From: http://support.microsoft.com/default.aspx?scid=kb;en-us;217190