I'm trying to play a DTS 5.1 movie via the Jellyfin app using directstream, but I'm not hearing any audio. Is the device not capable of downmixing the audio? When I play it via my android phone it seems to work.
I'm not having luck with downmixing to stereo within Jellyfin either. Probably some incorrect setting on the Jellyfin server.
Setup: I'm using a MiBox S that plugs into the TV via HDMI (non ARC), and and a 2.0 soundbar that plugs into the TV via optical cable.
What model TV? The 2020 models are all starting to drop DTS support on optical output. I think DTS is going away soon thanks to Atmos.
The server should transcode the audio if not and your audio system is not DTS compatible you won't get audio. Also if available turn off passthrough on the Client app. That way the server will know to transcode all surround audio tracks.
Only specific TV models can send DTS audio thru the TV and out again via ARC.
DTS support is appearing to become less popular with TV Manufacturors due to DolbyVision and online paid for video streaming Apps using DD / DD+ / Atmos for audio.
See here about DTS / TV audio support and configuration:
https://www.rtings.com/tv/tests/inputs/5-1-surround-audio-passthrough
Have you changed your tvs audio Output in the settings?