There are ways to enable Audio through HDMI on certain Nvidia cards on Hackintoshes, so it is quite likely these methods would work on Macs too.
They are tedious installs and frequently need to be redone with each OS update.
I haven't bothered to decipher them.
The good news is that a 2009 Mac Pro that gets the 5,1 flash suddenly remembers that it can do Audio over not just the DP, but also over HDMI on a flashed 5870. Is it full Dolby 7.1? I have no idea. But kinda neat that a port that doesn't exist on Real Mac 5870 works on flashed ones. Perhaps they developed 5870 drivers on Reference PC board and just left the drivers in?
Had Apple or Nvidia enabled Audio over DP on the Quadro 4000 than we might have easier ways to enable Audio on newer cards. IIRC, the Quadro 4000 does NOT have Audio.
Basically, while there may be LOTS of cards that have Video drivers in the OS, for whatever reason they don't bother writing or including Audio drivers UNLESS they have a need.
I think there was a Mini with an Nvidia card and HDMI. Using those drivers MIGHT get it to work on other Nvidia cards but the device id of the Audio parts of these cards are hard coded. So you would need to switch it out anywhere it appears in drivers. Good luck.
Mac Nvidia video drivers have always been 2 display channel only. Will be very interesting how they deal with new GTX680, if and when drivers appear for OSX. (They have released a Linux one)