[kolektor]

Originally Posted by neil_swann80

You could use a simple web tool like this:
https://audiotrimmer.com/audio-speed-changer/

Thanks for the tip. However, this site converts one file at a time and audiobooks contain many files. So this solution is out of the question.
But following this path, I found a better solution, I will share the link for others who know a bit more about linux/windows.
https://gpodder.github.io/docs/time-stretching.html

However, I am waiting for the inclusion of saving speed/pitch settings in the next version of the pocketbook firmware.