Ok, as I suspected pulseaudio doesn't know about your HDMI audio. This sounds a lot like this kernel bug.
Additionally this blog post seems relevant.
Many many thanks for your helping and interest, Geoff
Originally Posted by Geoff
Yesterday I updated kernel to 3.9.0 and not I can select "HDMI sterereo output". I have no option for 5.1, but I don't know if it's normal or not.
In any case, HDMI output works. I didn't try a newer kernel version since I don't know it thoses will work of with AMD fglrx drivers.
Many thanks again.