With a few days off, I started messing around with the surround sound in our living room, as we have in-ceiling speakers that were installed by the previous owner. I hadn't done much with it besides a quick initial setup when we moved in. I have an older Sony audio receiver with just the option for 2-channel or optical in, so my ATV3 setup is HDMI from the ATV3 to the television, then optical audio from the television to the audio receiver. I was pretty sure 5.1 sound was working well based on movie playback, but I wanted to be sure, so I found a Dolby .mp4 video file to test the sound, imported it into iTunes on my computer, and streamed it to the Apple TV. When the video was supposed to play on the back (surround right and left) speakers, it never played just in either of the speakers when it was supposed to. But I knew the speakers were wired correctly because the receiver can produce its test tones correctly in each speaker. Finally after some searching, I found Home Theater Speaker Check for iOS, fired it up on the ATV3 via Airplay, and the speakers performed as expected. So of course I'm happy that I'm getting 5.1 sound through the AppleTV3 (at least where supported by Apple, such as movies rented from Apple). So technically, problem solved, and I should just move on with my life. But my curiosity needed to know why the videos—provided by Dolby's website—wouldn't play the 5.1 sound correctly. It seemed almost like the stereo was taking 2-channel sound and making it work for the 5.1 system, but the receiver indicates that it's receiving a Dolby Digital 5.1 signal from the Apple TV during the Dolby test video. (When I played a video via the ATV3's YouTube app, it indicated it only was receiving a stereo signal, so I think I can trust the receiver to be correct about what kind of signal it is receiving.) Since the speakers all performed as expected when tested using the Home Theater Speaker Check app, I think it has to be an issue with the actual videos, though there is no way for me to know for sure. The main test video I was using had "Dolby Digital Plus" audio. I read somewhere that for AppleTV to stream 5.1 sound from iTunes on a Mac using home sharing, it had to be AC3, so I used Handbrake to create a new file with AC3 5.1 sound, with the same result. Here's the QuickTime inspector for each file showing the audio information: And the one I converted to AC3 audio via Handbrake: So finally, to my question: what type of audio does a video need to have to utilize 5.1 sound on an AppleTV 3rd generation? And where can I get a test video that actually uses that audio, since YouTube on an AT3 does not support anything beyond two-channel sound?