AirPlay video from retina MacBook Pro to Apple TV often doesn't send audio
This is a long-standing bug in the software with OS X Mountain Lion, that unfortunately Apple has not acknowledged or addressed. The simplest solution is to restart, but I don't like to restart my computer simply because AirPlay is broken.

Instead I do the following, which fixes the issue nearly 100% of the time, without a reboot:


Turn off AirPlay Mirroring
Open Terminal.
Type in the following commands (pressing return after each):


sudo dscacheutil -flushcache

sudo killall -HUP mDNSResponder

You should see your AirPlay icon disappear and quickly reappear in your menu bar during this process. After that, fingers crossed, all should work again. I stole this fix from the Apple Support Forums and it has worked well for me.