Ok, so here's the deal... My sound card has a digital audio jack (and should utilize 4.1 channels), which looks just like a common 1/8" jack. But my subwoofer (which acts as the surround sound decoder) only has an optical and coaxial digital input. So, I'd like to get my 4.1 digital channels rather than simply the analog stereo channels that I use now, but I can't figure out how to get the 1/8" digital audio output to the optical input on the sub. If you've got any suggestions, I'd really like to hear them.

You can't convert it to optical, you need to convert to the the bigger RCA coaxial jack. You need a 1/8" to RCA converter.

Is there anything special about this connector, or should I go pick one up at wal-mart? I assume that it should NOT be a stereo connector (which would split into a L/R RCA plug), but a mono connector?

Edit: I just tried with a Y-connector (stereo), and it only half worked. If I had the left channel RCA connector plugged into the sub's digital input, I could get an output for the right speaker and subwoofer only. If I had the right channel RCA connector plugged into the sub's input, I didn't get any output.

This leads me to believe that somehow, the information for any speakers other than the right is not being sent from my sound card (the sub might just filter all channels for low frequencies).

So I think I need a little more testing, if anybody knows of a program to output noise to all 4 of the channels, that would be a big help. Or if someone wants to just tell me how to fix this, that would be super as well.

looks like just a regular mono rca adapter to me
http://www.ramelectronics.net/html/RCA-mini-spdif.html

A stereo connector will work fine if you don't mind that one connector will dangle. One channel will send all the digital information. Keep in mind that your sound card can only mix digital stereo sound, you can only get digital 5.1 from streaming digital audio off of dvds.