Selecting iPad As Audio Source When Mirroring to Apple TV?
This isn't possible with how AirPlay currently works. It's a great idea to send to Apple for future consideration, however.

On my Panasonic Viera theres a headphone jack and a seperate volume setting for the headphones (so you can turn the TV right down). I've also got a set of TDK wireless headphones which do double duty (TV, iPad, iPhone). They've got line-of-sight range of about ten meters so you can move around without causing disturbance. Smaller TV's generally dont have audio out, but if you're looking to watch stuff in bed, why would you need anymore than the iPad fullscreen.