I have my AppleTV plugged into my receiver using a toslink cable and the receiver is Dolby Digital 5.1 compatible. ATV Flash and media player are updated to the most recent versions. All of my media is on a network share, and all of it is 720p x264 video, with AC3/AAC 5.1 audio, in a Matroska container. I have the AC3 enabled in the media player settings, DTS is disabled, however I don’t have any media using the DTS codec. The AppleTV sound output is set to auto.
For the most part all of my media delivers a true dolby digital 6 channel signal to my receiver as expected and it sounds fantastic. However I have come upon a couple of movies that do not deliver a dolby digital signal at all, 5.1 or stereo. Instead all I get is a standard stereo signal.
I have inspected these movies with VLC on my computer and there definitely is a 6 channel AAC/AC3 stream included inside these files. I can’t seem to find any solid difference between the movies that do output DD 5.1 and the ones that don’t.