Can't hear youtubes videos on non-youtube sites
 

I think it is for a month now, that I can't hear the sound of youtube videos anymore in non youtube-sites. It makes no difference when I turn the volume up. Before youtube upgraded their player, the sound was by standard muted.

It can't be because I used firefox plug-ins to save youtube files, can it?

Give this a try, go here (Flash Control Panel) and either delete all or just the setting for youtube.com.
This will reset your Flash settings for the deleted websites. Then try to connect again. If that doesn't work, uninstall the Firefox plugin and see if that helps.