Apologies. My initial reply was for multi-channel sound in movies, not music.
Unfortunately, I don’t have good news.
The Plex AndroidTV app does not handle multi-channel music. It plays the right and left channels only, discarding the other channels (no mix-down to 2.0, etc).
It is not specific to the Amazon FireStick. It will happen with any AndroidTV device. I have an Nvidia Shield and it does the same.
Also, all music is sent at 48 kHz, so any hi-res files are downsampled before playback.
There are several threads on the forum about this topic and currently no good resolution. Switching to a different Plex client may not solve the problem, as I found a thread with Apple TV users with the same complaint.
The only way I know of to play multi-channel music in Plex is via DLNA. You enable the DLNA server in Plex Media Center, then use a DLNA enabled client (not the Plex app) to stream the music.
I’m using VLC on my Nvidia Shield and it is passing 5.1 FLAC (48/16) to my Denon receiver as PCM. A 96/24 stereo file is downsampled to 48kHz according to the Denon app on my tablet. It does not provide the bit depth.
The DLNA client in the Denon was a mixed bag. It played 96/24 stereo at 96/24, but would not play my 5.1 FLAC file.
Again, wish I had better news. Unfortunately, Plex is still very much a work in progress when it comes to multi-channel music files.
My setup:
Nvidia Shield <-HDMI-> Denon AVR-X4300H <-HDMI-> LG B7 OLED
Plex Media Server on Synology DS918+
