As most everyone else has said, likely an issue of not having the drivers installed. Surprisingly, once in a while you can get lucky enough to have the drivers installed automatically if you do all of your windows updates from the actual windows update website rather than just waiting for it to tell you there are updates that need to be installed. So that might be worth a shot before trying to scour the net for the specific drivers.
Just go to the windows update site and click on the custom button instead of express and it will list any hardware/software updates available, sound drivers usually being one of them.