Apple TV - 44.1 kHz output
I have read in multiple places that the Apple TV outputs in 48 kHz; however, looking at my Audio Midi Setup on my new MacBook Pro the only listed option is 44.1 kHz / 16 bit for AirPlay to the Apple TV. The same option is available for the Apple Airport Extreme AirPlay output. Ironically, there are options to increase the bit depth for the internal speakers.
Maybe things have changed as I have a new Macbook Pro and Apple TV.