I have the same problem (affecting both firefox and Chrome) after upgrada to Kubuntu 11.10. I have tried HTML5 video to rule out dependency on Flash Player and, indeed, the video played fine (tested using http://camendesign.com/code/video_for_everybody/test.html) but no sound at all. By the way, the sound is working in other applications.
I have the same problem (affecting both firefox and Chrome) after upgrada to Kubuntu 11.10. I have tried HTML5 video to rule out dependency on Flash Player and, indeed, the video played fine (tested using http:// camendesign. com/code/ video_for_ everybody/ test.html) but no sound at all. By the way, the sound is working in other applications.