A couple of things have sprung to mind so here goes...
1.Have you inserted the jack-plug from the computer soundcard in the right output slot at the back?
2.If yes,have you inserted the jack-plugs into the right inputs at the back of the amp?
3.If yes,have you set the amp to select either "auxiliary" (or appropriate input)in order to get the amp to send the signal to the speakers?
Let me know if this works as I'll be here until 5ish.
Big L 266