or uninstall the sound software and re-install
if you have onboard sound the chances are the software will be within windows or on their update site.....
reboot your pc and when it restarts it should pop up a new software found box......use the recommended option of trying to find the drivers...it will search windows for them.
if no drivers found that way....go to windows update and then see if it gives you anything to download within the hardware section of windows update.
if there is a sound driver update in there...get it.
if you have a seperate sound card (like creative sound blaster, etc)...then go to the appropiate manufacturerrs website and then check in their driver/support section for your soundcard driver....download and install
if that fails...you may possibly have a faulty sound chip or sound card