When I run firefox and opera through process virtualization (sandbox) no sound came when playing youtube or any other videos.
Check out this post by one of the Avast Team as it acknowledges that they have found the source of this problem. It has been fixed internally at avast and will be in the next program update, I don’t know the schedule for this update.
http://forum.avast.com/index.php?topic=61398.msg526853#msg526853
So perhaps you can add your OS detail;s and setup to this topic.