For those that are having problems playing Dolby Digital content back on your Amazon Fire TV (i.e. you get no sound, but do see video). According to Amazon tech support, the Fire TV only supports Dolby Digital audio playback in MP4 container formats. So if you use any other container, MKV, M2TS, etc, you won't get audio played back correctly by the device.
I have created a FAQ with some possible work arounds for you to get the audio working, none of them are really ideal, and all are a pain to have to do. Unfortunately if you want surround sound and not stereo with your Amazon Fire TV and your personal video collection, then you will need to use an MP4 container instead of MKV or M2TS.
More information here: