If the issue persists I would suggest you to try uninstalling and reinstalling the sound drivers. Follow the steps to checking the sound drivers in device manager:
a. Click Start, and then click Control Panel.
b. Click System and Maintenance, click System, and then click Device Manager. Note If Control Panel is in Classic View, double-click System, and then click Device Manager.
If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
c. In Device Manager, expand Sound, video and game controllers. Now you may right-click the listed sound driver. Here you will get all the option to update or Install/uninstall drivers. Click on Uninstall. Check if at all there is an exclamation mark along the driver, it suggests issues with the driver installed.
d. Restart the computer. This will let the computer automatically detect the device drivers on restart of the computer.
Refer to the below mentioned link for guidance – http://windows.microsoft.com/en-us/windows7/Update-a-driver-for-hardware-that-isnt-working-properly
Hope this helps.