SOLVED: AC3 Mono won't play or transcode unless app is set to downmix to stereo

[coffeetablecandle]

On the AndroidTV client downmix to stereo only supports a few codecs:

Codec.Audio.AAC
Codec.Audio.MP2
Codec.Audio.MP3

Anything besides these audio codecs with more than 2 channels will trigger the audio stream to transcode. Since the video stream is compatible, you'll trigger a Direct Stream (Video copy with the audio transcoded to mono AAC). Which is why it works when downmixing vs Direct Play. Subtitles have nothing to do with your situation. It's a container issue.

It's all too complex for me maybe haha.

I guess what I meant was that I don't understand why downmixing to stereo (and therefore transcoding) seems to fix (or bypass) the container issue that prevents Jellyfin from playing the video directly.

In the end it doesn't matter I guess, I'll just have to be content with needing to downmix to stereo, or remux all the affected files, though I should first figure out how to avoid ERR0B1.