This can sometimes happen if a setting on your computer has inadvertently been changed.
Here are some things for you to check:
On the video window, along the bottom bar on the right, you will see five vertical bars.
The five vertical bars are the volume indicator in the video window. If the vertical bars are grey, it means the sound on the video window has been turned off.
If this is the case, use your mouse/cursor and click on the far right bar. This will turn the bars blue, and the volume will be restored.
If this doesn't help, we recommend checking all of your volume controls on your computer.