On 1.0.25 I’ve seen some audio issues where if at the initial screen I pick HDMI and play something from the firestick there. Instead of any audio, there’s just a clicking noise and then nothing.
Audio works fine from internal
Audio also seems to work fine if I first go with internal mode and then use the menu to pick HDMI
one time I had to change some sort of audio setting and then it worked.
Have you tried using other devices to connect via HDMI?
Just to eliminate the possibility that it’s a problem with your external source and not the PPM itself 
The audio for the firestick has to be changed to stereo / PCM … not sure of the exact wording
Ah yes, this could also be it. I used an audo splitter at home and I had to set the audio on firestick to make it work otherwise, it’s all just noise. Try which audio setting will work.
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.