Have you tried your sound settings, if it doesn't work with a particular computer?
Check your Vent settings to see if the "input device" is correct (For me, I have the choice between AC'97 or USB Camera) - this may have got changed somehow.
Also, I've seen some people (incl. myself) having issues with AC'97 cards. Sometimes the software (should appear as a blue square with yellow ripples coming out of a point, if you have it) messes up which connector it's going to see as which, so the pink socket gets assigned to something else. I'd say look through all sound manager software you have (Most will appear under Control Panel) to see if you can reassign the connectors.
(In my case, it needed to be set so that it was all "UAJ Automatic", but the AC97 drivers vary hugely from card to card)