Comment 6 for bug 400820

Some more findings:

Htop shown me too that pulseaudio use ~15 % of cpu. So, in a console i run : sudo dpkg-reconfigure pulseaudio. This command stop & restart pulseaudio.

And the good surprise now: there is no more over eating cpu activity:
 -no more trouble with previous gnome-volume-control-applet
 - nor with pulseaudio
 - and i can now open system/preferences/sound

So, it appear that it's clearly a problem of pulseaudio or of one of its dependencies