Did you check if you sound card is correctly detected ? If yes, it may only be a problem with the 1/8" Jack you insert for listening on the back of your computer. Check if you can plug a front panel :!: for your audio connector, so that you plug it in front with a wire that go on your motherboard, there is usually a place for front panel audio.
However, If your card is not listed in the PNP device at startup screen just after the bios load (presuming you dont have a fullscreen bootlogo, you should see "Multimedia audio controller" when you boot) then the card stopped fonctionning completly and it's looking bad