No signup, no ads (albeit maybe analytics and crash telemetry), GPL3 code, fairly minimal and keeps audiobooks away from messing up music playlists or from podcast app complexity. Haven't used it enough to give a full review, but it seems exactly what I wanted and good for Libribox book downloads.
[1] https://play.google.com/store/apps/details?id=de.ph1b.audiob...
[2] source: https://github.com/PaulWoitaschek/Voice
https://github.com/PaulWoitaschek/Voice
For lectures on youtube there are Firefox extensions.
I use an app called "Voice" [1] on Android for audiobooks. I'm so used to it that any other app or way to consume audiobooks seems below par.
I don't really use audible, but if you looking for good audio player on Android here is one that can do this:
https://play.google.com/store/apps/details?id=de.ph1b.audiob...