Dolby True HD + Dolby Atmos 7.1 not working on LG TV App

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Ok, so i installed the Jellyfin app on my LG CX 4k OLED, but i cant get any 7.1 Dolby True HD to work via the LG app. I have a SONOS surround system connected via HDMI ARC.

Make sure that your Sonos supports TrueHD Atmos 7.1 via eARC. Not just Atmos -- TrueHD Atmos 7.1. It could honestly be as simple as incompatible cabling, but based on having no issues with Netflix, that's not likely.

Some off the Movie files have Subtitles. When i select any Subtitle it starts transcoding and the whole picture changes. (colors are off etc.) It says Subtitle/container CODEC not supported. (HEVC Main 10, MKV container).

Image-based subs are not supported by the webOS app. This makes sense -- the media was being tonemapped and you likely got AAC audio out of this.

Is it just bad/wrong files or what am i missing?

It's gotta be something in the LG settings. I just went into my C1 and there are a TON of audio settings, several of them for sound bars. Both TV Sound Mode Control and Sound Bar Mode Control seem possible, Select HDMI Input Audio Format maybe, I'd turn off LG Sound Sync, and try with both eARC on and off. Note that almost all of these settings refer to an **LG Sound System** so may not be compatible with your Sonos.

This doesn't appear to be a Jellyfin issue, but I'll reserve judgment. You'll probably want TrueHD enabled in JF to ensure the audio stream isn't transcoded. From there, trial and error.