This happens occasionally when you use an external monitor connected via HDMI as this interface also plays audio but your monitor does not have speakers.

To resolve in windows 7 and 8:

  • Go to Control Panel and open Sound.
  • Under the Play Back tab, select Speakers.
  • Click the Set Default button at the bottom right.

This is known to happen occasionally when laptops are being disconnected and re connected to HDMI frequently.