You need to check a few things, because the no sound issue could be caused by:
- Speakers are set on mute.
- Wrong jack configuration in the sound panel. You need to assign each jack to the correct color and configuration to hear properly.
- Sound driver is faulty. Go to Device Manager (Start > Run > devmgmt.msc) and re-install the driver. Right click the sound device and choose Update Driver.
After this procedure, access Uninstall a Program and remove the Facebook Video Calling, then re-install it.