check @ DTS-HD, TrueHD, ATMOS, showing as Multi-channel on AVR
this shows working audio pass through settings, if your AVR supports truehd/atmos, then that should be working without transcoding any audio.
maybe the hdmi cable is too old, I dunno.
enable/disable pass through in the plex settings
volume control: use fixed for pass through, use other option for non-pass through
you can also disable passthrough via the same settings above
your setup should be like
shield <> hdmi 2a cable <> atmos receiver <> hdmi2a cable <> 4k tv
