The sound card might be disabled or not working properly.
Check your sound card adapter through the Device Manager:
1. Right-click on My Computer icon, choose Properties.
2. Click on Hardware tab.
3. Click on Device Manager button.
4. Look for sound, video, game controllers.
5. Click on the + to expand list.
See you if you have any question mark or exclamation mark on a device on that list.
And what type of sound device do you have listed under there?