Well as you have seen yourself, MC can play these files, as you have found out by renaming them. As I've mentioned before, renaming them does not change the structure of the file. The name is often meaningless, and may just be used to link to the right application, so giving it any name that MC knows as audio would make it work in this case.
I do not know what format those thd files actually are in, however. Maybe raw truehd? That would be a terrible format, audio should never be "raw". Maybe mp4 or mka with a weird name? Who knows.
Unless its some variant of mp4, its unlikely we'll be ever able to tag them. I can teach MC about the file extension, so playback works without renaming, however.