SPDIF is a digital output, you can't send it out to a speaker.
SPDIF is also called coax digital audio
You have to plug that sub into an analog output.
The output that your normal left and right speakers are comming from is analog. You can buy a jack splitter and run it that way, with an adapter to go from mini headphone jack to coax RCA.
On most AC97 devices, you can assign the microphone or the line-in jack to output different digital channels, but there are countless different drivers and menu layouts so there is not going to be a quick guide to tell you where to go.
