Right click the volumn icon in your system tray. Select "Adjust audio properties". Select "advanced" in the speaker settings. Click "down" arrow and select your speaker system in the drop-down menu.
b4, i got it working, i had 1 plug out, every time i touched the case, my sub would pop, so if jayare012 suggestion works, try pluging the unused plugs into the mic hole.
but it sounds like the exact same problem i had