For HD bit-streaming, try this:
1) nVidia Control Panel -> Set up digital audio -> Turn off audio (under HDMI from dropdown list) -> Apply
2) Select your AVR from the dropdown list -> Apply
3) You should see the supported formats under Sound properties
You may have to try it a few times...
I might have a potential solution to your issue. YMMV.
I have a GTX 670 SLI setup with 3 x Dell U2311H and an "accessory" monitor for HDMI audio output. My connections are as follows:
1st card: Top DVI to left monitor
Displayport to middle monitor
HDMI to AVR, which outputs to the DVI of...