Ok i had this once too. i uninstalled all sound drivers ( from the registry too.) open start menu type regedit in the run dialog box. your looking for folders that are made by your card manufactuer, eg, Creative.
Once done try disabling any onboard sound through your BIOS options. reboot re-install sound drivers and see what happens.
Sry if you have already tried this. im just assuming that you have not!
Also what type of card is it 5.1 7.1 external or internal?.
Also..plz dont think im calling you stupid......is the jack in the right socket i ask this because some card makers dont color code or show which jack should go where sometimes, i have come across this myself.
Hope this helps in some small way.