No audio when AirPlay mirroring from MacBook Air to Apple TV Air running Yosemite, played a video and turned on AirPlay mirroring to my Apple TV 4 running tvOS 9.1. The sound keeps coming from my Air, not the TV. I checked the Sound.prefPane and confirmed that the Apple TV is selected under Output. I've tried restarting the video as well as disconnecting and re-connecting AirPlay Mirroring, but the sound always comes out of the Air.
Found a solution on Apple forums, posting it here in case anyone else runs across this problem. I had to:


Turn off AirPlay Mirroring
Open Sound.prefPane and select Apple TV as the output
Turn AirPlay Mirroring back on


For some reason it appears you have to manually set the sound output before connecting AirPlay Mirroring

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