Go to the Device Manager, RIGHT click to higlight the RealTek audio device and then LEFT click "Uninstall". DO NOT check the box to also uninstall the drivers. Restart the PC and when Windows starts it will detect and reinstall the sound. This fixes a lot of problems and a first step to fix your problem.
Shortcut to Device Manager, Hold the Windows key down and then press the Pause/Break key. In the panel that comes up, Device Manager is in the task list in the upper left.