https://www.w3.org/TR/epub-a11y-11/#sec-mo-complete

Meeting this Objective
EPUB Creators MUST provide synchronized audio playback via Media Overlays for all textual content.

This statement could be interpreted as saying to provide synchronized audio using the Media Overlays mechanism, and not to use another mechanism.
Since the main purpose of this clause is to say that the complete audio narration should also be provided along with the full text, we believe that "for all textual content" is a weak explanation and should be "for complete textual content".