First, try running the Fix It utility provided by Microsoft. It will diagnose and repair all major sound problems automatically. Also, make sure you don't have a HDMI cable plugged to your computer, because Windows will consider it the default sound device. After that, perform a full system scan using your security program and run a registry cleaner from Software Informer database. If the problem persists, you can get in touch with a Realtek technician using the contact information from the official web page.