[rizu]

I don't think real pass through is possible with the current version state. What comes out as DD or DD+ is re-encoded by the stick, I came to that conclusion when trying to play a 7.1 DD+ and it came out as 5.1.

After many tests this is the config that worked to get a pure 7.1 PCM stream.

Fire TV Audio setting : Best Match

Kodi Audio Setting :
Audio Decoder
  Audio output device : AudioTrack(IEC), Kodi IEC packer (recommended)
  Number of channels : 7.1 or 5.1
  Output configuration : Fixed (this is important to force the PCM stream)
  Limit sampling rate (kHz):  96.0   (this is important to force the PCM stream)
  Send low volume noise: OFF  (this is important to stop the Fire Stick from crashing and rebooting)

I used a while these settings and while the audio quality is notably better but it justs FireTV crash and reboot all the time. And yes I've set that low volume noise off as well. I had to go back to DD+ encoding setup but I really dislike it too as I feel everything has audio desync with it. Didn't observe the same when Kodi actually decodes everything and sends PCM to receiver - it's just too unstable for daily use on my end.