did you check the setting in the Flash player that is used to make sure that the volume isn't muted?

Also check the volume setting in the Window Volume mixer.

• https://support.mozilla.org/kb/fix-common-audio-and-video-issues

I can't find any setting in neither 'settings' dialog (small panel and General Settings) that will allow me to mute the sound. Where do I find this? Hardware acceleration is switched off (I've read on many a forum that having it on, may cause this problem - and I notice no difference in playback quality).

In the Windows Volume mixer panel, I can see both the Firefox and the Firefox plugin container volume controls, both ON and set to max. level. The funny part is that when a Flash video is playing, I can see the volume level in the Firefox control moving up and down - so apparently the OS *is* detecting sound, it just isn't channeling it in the right direction. Which it *does* when I switch the Protected Mode back on (and restart the browser); I fail to see the connection. But then the plugin crashes every 10-15 minutes.

Also, this problem had never occurred before the latest (auto-)update of the Flash plugin to 20.0.

Never mind - I found the perfect solution. I am using Opera from now on. No problems whatsoever and far less arrogant programmers. No more worrying, at every single update, about what they will have f*cked up this time. Thanks for your time, tho. Much appreciated.