Looks like someone else may have found a workaround!
The important bits:
This solved my question by jonvid on Aug 5, 2012 11:42 PM
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.
I almost have the same problem. I cannot move the sound from my MacBook Pro to the TV while doing a FACETIME call. I tried the above workaround ("...BEFORE turning on the mirroring, change the output audio on your computer to Apple TV in System Preferences...") but it didn't solve the problem. Then I tried a SKYPE call, and the sound moved over to the TV automatically. So my problem is peculiar to FACETIME only. Any suggestions?