Airplay - MacPro Stopped Working

I have an Early 2008 Mac Pro and a new Apple TV. Audio output, via Airpay, worked just fine from my Mac to the AppleTV until I just tried it again today!

I tried restarting the Mac and the AppleTV. The option appears in the Audio Output on the Mac, but when selecting it, it doesn't send audio there and defaults back to the internal speaker after a min or two.

I'm at a complete loss. Airplay is working just fine on my iPod to AppleTV, so I'm guessing its a problem with the Mac, possibly since updating to Mountain Lion 10.8.2. Seems lately most of the Apple Updates break something somewhere.

Is there any way to "reset" the audio system on the MAc? Any preferences to possibly delete?

Thanks.