Comment # 2 on bug 961143 from Stakanov Schufter

Thank you that comment helped me a lot to see. 
It is a javascript problem.
If I open youtube, (I do not have flash installed) it plays the videos and the
sound and in the pulseaudio streams appears cubedetutils (or something
similar). 
So that seams to be a problem of javascript and FF when reproducing these kind
of soundfiles (mp4/mp3). 
I do not have vlc installed and use gstreamer.

Funnily it does not complain about anything. Just does not play it.