Just on ‘youtube’ or is your volume icon missing on your computer, too? I had this happen on Windows 8.1 ... no volume control at all & I first noticed it when I tried to play youtube videos.
If this is your problem, here is the “fix” that worked for me:
http://www.sysprobs.com/volume-icon-missing-on-windows-8-or-8-1-how-to-get-it-back
More detail - I had to go to method “2a” ...
Thanks. I switched my browser from Internet Explorer to Firefox and now I have audio.
The speaker icon was missing on my computer, too, but I rebooted and I at least got that back. I switched to Firefox as my browser so I have audio on youtube when I’m on Firefox but not Internet Explorer.