Ok, we managed to get a sideloaded audiobook to work on these Kindle Fires. Download Astro Player Nova (the pro model is free). Load your mp3 audiobook into the SD Card. It will find the audiobook and play it just fine. Still working on getting text-to-speech to work beyond the 2nd page. Not sure if we have the right app for it or not. There might be better audiobook players, but getting them to load on the Fire is a challenge. Many seem to be blocked or simply not available. It's a shame the Audible player won't do it. Plays all my Mp3 audiobooks on my Ipod.
Suggestions are welcome. I find also that some apps that work fine on my Fire HDX do not work on the $49 Fire (what DO we call that one? Is that a Fire 7?).