I have some ripped BD movies in a mkv container.
The XBOX Plex client does passthrough TrueHD audio of half the files I have. Analysing the difference among them using MediaInfo, I found out that in the ones which worked, the TrueHD audio track was always the first ID in the audio track list.
So, using MKVToolNix I simply switched the order of the audio tracks, ensuring TrueHD track went first always. And to my surprise it WORKED! Passthrough happened flawlessly with every audio track.
All I need the client to have now is support for native image-based subs.
I hope it helps.