mmmh... i don't mean to make you sound stupid.. but did you check to make sure u didn't hit mute my mistake?
i've done it...
My does that alot and it somehow always ends up on mute. Sometimes just restarting the computer brings it back on.
You may need a new soundcard. however there is other option but it would take a while to explain what you have to do.
you said....what? huh? try again?
Problem with your sound drivers, change them. Look for CD which came along the PC or scearch it on net if you know it.