I also had the problem with the fire TV stick 4k today. There was just no audio I had to go to Settings->Audio->Audio->Sourround and select stereo.
It seems like the fire TV stick 4k does not automatically recognise to only send stereo but instead sends dolby digital signals. Strange thing is, that when I connect my old fire tv box (2nd generation) there the automatic selection of the needed sound format works. I’m not sure if something can be changed in the PPM. If not I recommend to add it to the Guides section.