The Storyteller app was brought up earlier, but it's worth mentioning again since it's progressed quite a bit.
https://smoores.gitlab.io/storyteller/
Storyteller is both a self-hosted app capable of *producing* EPUB3 files with Media Overlays, if you have the eBook + Audiobook pair already.
It is also a mobile app that is provides a great reading / listening experience for EPUB3 with Media Overlays. The files can be downloaded from your self-hosted instance of the Storyteller web app *OR* loaded directory from your phone's local storage, so it can play any EPUB3 file.
iOS:
https://apps.apple.com/us/app/storyt...r/id6474467720
Android:
https://play.google.com/store/apps/d...es.Storyteller