Apple do not themselves support sending audio via non-original video cards even though the card may be capable of this!
This artificial crippling of video cards by Apple is extremely annoying and does not enhance Apple's reputation, it is made particularly absurd by the fact that if you boot in to Window via Boot Camp using the same Mac, the same video card, the same cable and the same display it works straight away with all features enabled.
Not only does this apply as in this case to audio support, but it also applies to support for 30bit color.
While it seems currently still impossible to get 30bit color to work in OS X with video cards in a classic Mac Pro that can support this, there is a way to get audio to work. A kind person has written a kernel extension which enables HDMI Audio and this I believe also works for Displayport Audio. I use it successfully with an AMD Radeon HD 7950 card to get audio out via HDMI.
See http://forums.macrumors.com/threads/guide-hdmi-audio.1499797/page-15#post-215990 93
Note: My AMD Radeon HD 7950 is also capable of 30bit color as I have proved both by checking AMD's specs and doing so successfully via Boot Camp in Windows 10. However Apple as mentioned have wilfully crippled their OS X drivers.
You will have to enable developer mode in Yosemite to allow loading an unsigned kext, in El Capitan you will need to at least temporarily disable SIP while you install the kext.
