Here’s a temporary workaround for the Youtube / no sound issue:
First, try clearing your browser’s cache and cookies, and/or try this:
The mute will keep coming back for each video you watch, but you can get your sound back each time by pausing the video, right clicking the little X by the speaker symbol and then clicking the speaker symbol itself. Then start the video again.
Youtube has acknowledged this ‘bug’ and should have it fixed soon.