its a radeon 6870 and I dont see hdmi audio driver in device manager all i have under sound is (AMD high definition audio device) and i have tried updating it and uninstalling it.
check in your bios that the onboard sound chip is still turned on. if it not turn it on and see if the sound comes back. if not go in the device manager and disable the amd audio. then reboot and see if real sound comes back.
also download and install the newest realteck audio drivers from realteck.
I'm having the same problem I just installed my GTX 980 and all my sound has cut off, the audio manager still recognizes my headphones but no sound plays through them. This happens with all my headphones/earbuds.