Based on your description I think the problem here is the receiver and not the HD graphics controller. The Blu-Ray player is recognizing the DTS which is supported by the Graphics controller.
I found something on the web similar to your problem that may be helpful for you:
Thanx Kevin. I read the article you directed me to. I understandl that you got my setup wrong. I used a blue ray player (sony) from my stereo set. That player workes fine, and displays hd-master. When i hook up mij computer, it workes fine with dts, it displays dts on the receiver, but if the movie is in hd-master the receiver uses multich. I would like the receiver to perform all of the decoding. And let the computer just handle the movie without decoding the sound. So i wanted te know if it is possible in any way to always let the receiver process the sound decoding. Something like a passtrough option.