It’s so hard to figure out ahead of time what is supported by this AirPlay stuff. Or at least it was for me it seems. So here’s what I know for some other lost soul stumbling through the damn wilderness one day.
The Apple TV (newest revision as of 01/23/2013) will play audio for music to wireless AirPlay speakers. It will not play audio for videos to wireless AirPlay speakers. The audio for your videos – Trailers, YouTube, etc – is passed through HDMI or the digital audio output connection.
The iPad (gen 3 retina), iPhone 5 and Macbook Air can each be mirrored onto the Apple TV and both sound and video will be passed. However, the audio will follow the same restrictions laid out above and will not be played through your wireless AirPlay speakers but through the HDMI or digital audio output.
I was able to get the Macbook Air, connected to the monitor via HDMI, to stream audio via AirPlay for the video I was playing, but the audio was about 4 seconds off and seemed to be causing the laptop a lot of pain.
Short version – only expect audio from music on any of your devices to play wirelessly. Do not expect audio from video on any of your devices to play properly. If you achieve a different result… please share!
My next step is to try one of these HDMI to HDMI + Audio adapters as I’m trying to do all of this without a real TV, just a DVI monitor that doesn’t support audio over HDMI. Fingers crossed.