You simply need to reinstall the sound driver. The setup file should be on the disk which was supplied with your motherboard (assuming, of course, that you've got such a disk and that you're not using a separate sound card).
If you've not got the disk you'll need to determine which motherboard you've got, and then download the driver from the manufacturer's website. (Again, that assumes that you're using 'onboard sound', and not a separate sound card).
The easiest way to find out which motherboard you've got is to download, install and run Belarc Advisor:
http://www.belarc.com/free_download.html
Chris