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.

[1] https://apps.apple.com/us/app/mp3-audiobook-player-pro/id889...

[2] https://play.google.com/store/apps/details?id=ak.alizandro.s...