OS X 10.8 Airplay sound fails

On my laptop, the Airplay refuses to ave sound. First it was just with some web content, now it's all sound.

Menu Bar -> Airplay -> Mirroring On

Result: Flawless video, no sound.

System Preferences -> Sound -> Output -> Apple TV

Result: Selection pops to the last item, Airplay stops displaying on TV. (Airplay on the mac thinks it's still working, but the Apple TV stops displaying.)

Rebooting doesn't help, pretty much nothing seems to have an effect.

Macbook Air

OS X 10.8.5 (all updates)

Firewall disabled

AirParrot installed but not active (has same issue, though)

Apple TV (all updates)

iOS devices work fine with the Apple TV

Video works fine. Just audio fails.