Click the "Start" button
Click on "Control Panel"
Double-click on the "System" icon
Choose the "Device Manager" tab
Expand the tab "Sound, video, and game controllers"
Look at the devices that expand under that
Double click on anything relating to sound (there might be multiple devices, do this for all of them)
Click "Update Driver..."
While updating and once done, click on whatever gets you out of those windows (ie. "OK" "Apply" "Yes" "Continue") you get the point...
Restart your computer
If that doesn't work, i have one more thing for you to try.