I can see when a video is playing but cant hear it

Go to the Control Panel and locate the icon for "sound and audio devces".
Select the tab marked audio and look for the tab marked "sound playback" look at the pull down and see if what device is connected (if any) if not select your audio driver (this could be listed as many different things ----a common one is "Realtec"). This should do it (i am assuming the sound works on other things. If this doesn't work select the tab marked hardware and make sure all are working and if not use the troubleshooter to repair