On iPhone we use "MP3 Audiobook Player Pro"[1]. Despite saying "MP3" in its title it supports many formats including OPUS. They broke OPUS speed-up and hadn't fixed it for some time but have fixed it recently so it's great again.
On Android "Smart AudioBook Player"[2] does a perfect job.
Not OP, but I have all mine stored on my plex server, and use Prologue to access them on iOS devices and like it a lot. https://apps.apple.com/app/id1459223267