[Mosu]

I'm asking about TrueHD as TrueHD and MLP are very similar. Both are two of the few audio codecs (RealAudio being another one) where you cannot start decoding from any packet. Therefore mkvmerge flags only those frames decoding can start from as key frames. Maybe mpv expects that all audio frames are marked as key frames — but that's just conjecture, of course, I don't know mpv's source code.