Hallo,
My system:

Windows 7 64bit
Asrock Pro3 Gen3
SAPPHIRE RADEON 6870
Intel Core i5-2500k
Corsair 2x4gb

I have the following issue:

I want to play games on my small LCD (LCD Hdmi plugged in to Radeon 6870) and watch movies on my big TV (TV plugged in to Asrock onboard Hdmi) without switchting the Hdmi cable when I want to change from gaming to watching films.
When my hdmi cable from my tv is plugged-in to the radeon 6870 sound is working fine. But when its plugged to the mainboards hdmi port the sound is disabled. Video is working but the sound isnīt. Windows says AMD Hdmi Output "AMD High Definition Audio Device" is not plugged in. I changed the BIOS Settings from Hdmi Onboard Sound "Auto" to "Enabled" but nothing changed.
I think SAPPHIRE Radeon is eventually supressing the onboard Soundchip.
Perhaps its not a big issue but i tried everythink i could think of. I hope someone can give me an advice.