[gentoo-user] OT: No audio while playing HTML5-video (YouTube)

Hi,

I am trying to get rid of Tyrannus Flashorus Saurus ...

But it seems, that this needs more than an impact
by a giant meteor...;)

When starting Firefox with a clean new Profile with no
Addons added, it is possible to /watch/ (watch != hear)
a HTML5 video played from the YouTube site. But there is 
nothing to hear...

The audio system of my Gentoo Box seems ok, since playing
a video with mpv from the commandline let me watch
and hear it.

The YouTube-Page, which checks Firefox for the availability of
HTML5 video cpapabilities says "OK! GO!"

I googled that topic and found many hints for Windows10,
-- and for Linux regarding gstreamer:


root> eix -I gstreamer
[I] media-libs/gstreamer
     Available versions:  
     (0.10) 0.10.36-r2
     (1.0)  1.4.5 ~1.6.1 ~1.6.2
       {+caps +introspection nls +orc test ABI_MIPS="n32 n64 o32" ABI_PPC="32 64" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.10.36-r2(0.10)(20:25:26 07/24/14)(introspection nls orc -test ABI_MIPS="-n32 -n64 -o32" ABI_PPC="-32 -64" ABI_S390="-32 -64" ABI_X86="64 -32 -x32") 1.4.5(1.0)(18:49:57 03/16/15)(introspection nls orc -test ABI_MIPS="-n32 -n64 -o32" ABI_PPC="-32 -64" ABI_S390="-32 -64" ABI_X86="64 -32 -x32")
     Homepage:            http://gstreamer.freedesktop.org/
     Description:         Open source multimedia framework

[I] media-libs/phonon-gstreamer
     Available versions:  4.7.2 ~4.8.2 **9999 {alsa debug +network +qt4 qt5}
     Installed versions:  4.7.2(05:01:23 10/13/14)(alsa network qt4 -debug -qt5)
     Homepage:            https://projects.kde.org/projects/kdesupport/phonon/phonon-gstreamer
     Description:         Phonon GStreamer backend


The referenced gstreamer-plugins (The good, the bad and the ugly) are
no longer listed with eix.

I have no clue, what to check next...

Any help is very appreciated, thank you very much in advance!
Best regards,
Meino