Can someone explain in detail how to select the sound output when I'm watching a movie over Airplay? I have a fancy speaker system hooked up to the thunderbolt port on my MBP. It's no problem to choose that system in System Preferences: Sound or by clicking on the speaker icon. But when I Airplay (via iTunes) to my Apple TV which is plugged into a projector via HDMI, the sound comes from the tiny projector speaker no matter what I have selected in Sound Prefs. If I do screen mirroring it works but the video is super choppy. From iTunes, it ignores my sound prefs.