Currently Being ModeratedAug 1, 2012 8:32 PM (in response to tdnorton)
Try changing your output sound setting to Apple TV. AirPlay mirroring only puts your screen to your TV I think. EDIT: heh nevermind I should have read your post in full before replying. Anyway, changing the sound out to Apple TV works fine for me.
Currently Being ModeratedAug 2, 2012 4:08 AM (in response to tdnorton)
Exact same problem I just found out as I'm trying out Mountain Lion for the first time.
Currently Being ModeratedAug 5, 2012 9:42 PM (in response to tdnorton)
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.