The 6850 supports multi-channel audio pass-through for Dolby TrueHD and DTS Master Audio over HDMI. I use a EVGA GTS 450 and my Yamaha RXA-1000 receiver does all the decoding including DTS MA and Dolby TrueHD. Works flawlessly. I only had to set the Nvidia driver to Audio over HDMI.
I guess there is a similar procedure for your card
UVD 3 now includes Multi-Stream Transport decoding for Blu-ray, which includes 3D, as well as hardware acceleration for DiVX encoded files. ATI HD Audio is unchanged, offering high bitrate multi-channel audio pass-through for Dolby TrueHD and DTS Master Audio, required for protected audio path playback of Blu-ray audio via your receiver. The audio output can be directed through either the HDMI or DisplayPort outputs, but not both at the same time. Using these audio configurations will require a supported video playback application like Cyberlink PowerDVD 10, and for the audio LPCM output to be set inside the application. Lacking the necessary Home Theater setup to fully test 3D Blu-rays and bit-streaming audio we have been unable to verify correct operation.