have you tried turning the volume up with the windows sound manager? or maybe the mute has been pressed? maybe the lead is faulty. if its an onboard soundcard, has it been disabled in bios? try uninstalling and reinstalling the drivers. try getting the most up to date driver set for the sound card. this might sound obvious, but is the soundcard connectted to the right input on the amp/output on the soundcard? and is the correct aux selected on the amp? try a different amp and/or lead.