just posting this in case someone else might need it.
If you place mp3 files in the "audible" directory of the kindle (sd or internal),
you can play them as audiobooks, just like "audible" audiobooks. Did it with a few LibriVox files, no problems.
Just to experiment I tried with a DRM free iPod audiobook (also from LibriVox).
These files have the ending m4b, and have ISTR acc encoding. The Kindle did not see the file, so just for kicks, I renamed it to have the ending mp3. The Kindle saw it, but crashed when I attempted to play it. More or less as I expected.
However, what happened next was unexpected. The Kindle would not react to a switch off/on. So, I reset it. The screen flashed black a few times, and that was it. I tried reseting again, I tried removing the battery for a while. No joy. Just the same flash sequence followed by nothing.
I believed for a few minutes that I had bricked the damn thing.
The solution turned out to be plugging it into my laptop via the USB cable, and reseting again. It came up, booting twice (showing the Kindle logo), before it eventually showed the main GUI, and not the "plugged into USB" display. My laptop got a bit confused, and also crashed.
After I rebooted my laptop all was well.
Sense of relief was profound......