Having a very similar issue, as noted in Sonos Arc soundbar DTS to AC3 Transcoding not working - #115 by jesalr
Shield 2017 → LG C9 → Sonos Arc
Disabling passthrough on Plex means DTS videos are coming through as multichannel PCM on the Arc (Great!) but so does TrueHD, which isn’t something I want converted as it drops 7.1 to 5.1 and also loses Atmos
If, instead, I change to HDMI passthrough in the app, I lose DTS audio (obviously) since the Arc can’t deal with that.
If I change the settings in Shield from automatic sound formats to manual, then disable DTS and DTS-HD (but leave TrueHD, Atmos, and all other Dolby formats) I still lose TrueHD in Plex
