Since there are reports about youtube audio being silent so if you use youtube.com and the audio is not heard then do the steps below if you use JAWS. Sighted users have it easy as they see visually the up and down arrows for the volume control but we do not.
Go to your youtube video you want to hear.
Go to the top of the page and hit insert plus z. You should hear JAWS
Now just keep tapping on the tab key until your on the mute button. Now
leave the tab key, and use the up arrow key to, hopefully, increase the volume.
When done press insert+z again.
Worked for me.