I have done this, and it still only allows stereo setup.
Did you find a solution to this? I have a similar setup:
Radeon 7870 --- HDMI --- LG AV Receiver --- HDMI --- Sony TV
The same setup with my PS3 connected instead of my computer outputs great 5.1 surround sound with no configuration needed. When I hook up my computer, Windows only gives me the option for stereo. It recognizes the "LGHomeTheatre" and the "AMD High Definition Audio Device". The video passes through my receiver to the TV just fine.