I have a 2011 MBP running Mountain Lion and the latest ATV. When I use airplay mirroring I can see the display on my TV but no audio. It works just fine if I airplay from itunes, just not when I have a quicktime movie playing and try to mirror my desktop. I even tried manually changing the output selection from internal speakers to Apple TV in audio preferences, but for some reason it wouldn't let me. It kept just switching back to internal speakers. Please Help!!
I had this same problem and I think I just found a solution. Turn on your Apple TV. BEFORE turning on the mirroring, change the output audio on your computer to Apple TV in System Preferences. After that, turn on the mirroring and the sound should be coming out of your tv.
Again, if you don't change the output before mirroring it will not work.
Hope this helps.