When I try to play audio books on my Kindle Fire HD8, the play button keeps "jamming" - it will play for about half a second and then stop. It will keep doing this each time I touch "play". The only way to get it to work is to switch off the fire and switch back on. Then the play button will work. But the next time I try, it jams again. I have tried with several audio books, I have tried with speakers, headphones and bluetooth. But the same problem keeps occuring. If I use a switch on my bluetooth headphones to play, instead of audio books it will play from my music files.
Before contacting Amazon and asking for either live support a simple free replacement of your Fire have you tried restoring Factory Defaults on the device? It should cure and serious software faults on the device.
Also have you also checked that it has downloaded and updated to the latest software?
On the HD8 both options should be available under the Home / Settings menu options.
One last thought - Have you got the HD8 in a case? It's possible the case or the magnets in the case are messing with the Fire - try it without the case and see if that cures it.