I know this is an old thread so sorry for bringing it back firstly but had to add a bit of info here seeing as this was one of the top results in google when i was searching for this perticular problem.
I went through hell for 2 days trying to get the HDMI audio working on my 7870, it was showing ATI HMDI Audio in playback devices but i could never enable it as 'Set as default device' was always greyed out. After much frustration howeverl i finally got it working and this is how:
1. Download and install the newest CCC if you haven't already.
2. Uninstall both Realtek drivers from control panel. Restart.
3. It seems silly but install both the drivers you just uninstalled again from here (the default and the ATI HDMI Audio Device driver): http://www.realtek.com.tw/downloads/downloadsCheck.aspx?Langid=1&PNid=14&PFid=24&Level=4&Conn=3&DownTypeID=3&GetDown=false. Restart.
4. Now you want to uninstall just the ATI HDMI Audio Device. Restart.
5. After the reboot windows will now automatically download it's own driver which is compatible with the 7870.
Now you should be able to go to playback devices and enable and set hdmi audio as default now. I hope this helps anyone having this problem in the future.
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