hi

have you run the troubleshooter for sound in windows 7?
Try using the Playing Audio troubleshooter to fix the problem. It checks for common problems with your volume settings, your sound card or
driver, and your speakers or headphones.
Open the Playing Audio troubleshooter by clicking the Start button , and then clicking Control Panel. In the search box, type troubleshooter, and then click Troubleshooting. Under Hardware and Sound, click Troubleshoot audio playback.* If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
Tips for fixing common sound problems >>>
Tips for fixing common sound problems