Try these:

• Upgrade your flash plugin

[https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix...]

• Use Html5 version of YouTube
• Reset your Firefox cache

[cache]

• Reset your Firefox

Reset Firefox preferences to troubleshoot and fix problems

• Maybe there's a problem in YouTube preferences!

Keithmt:

You appear to already have the most current Flash plugin.

See these articles for possible solutions:

• https://support.mozilla.org/en-US/kb/what-to-do-if-firefox-wont-play-any-sounds
• https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues

Other - You should check your Plugins and update as needed:

1. Check your Plugins - https://www.mozilla.org/en-US/plugincheck/
2. Adobe Shockwave for Director Netscape plug-in, version 11.5.8.612 (not the same as Adobe Flash/Flashplayer/Shockwave Flash)
   • http://kb.mozillazine.org/Shockwave
   • Download, save, close all browsers, run the installer that you just downloaded - http://get.adobe.com/shockwave/

Did you check the volume setting for the plugin container process in the volume mixer while a video is playing?

You can also try to clear the Flash local storage and settings for YouTube or all of them.

• Windows Control Panel > Flash Player, click Delete All and Delete Data

Flash Website Storage Settings panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

Global Storage Settings Panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html

Flash Local storage settings Help:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

First, thanks for the replies, I checked all of them. The Silverlight plug in needed an update which fixed the problem. Prior to that I updated and reloaded just about all the rest and emptied all the caches. Pretty much the last bullet!