https://w3c.github.io/epub-specs/epub33/rs/#confreq-rs-epub3-mo says:

Reading systems with the capability to render prerecorded audio SHOULD support media overlays [epub-33].

But then:

When a reading system loads a package document, it MUST refer to the manifest item elements' [epub-33] media-overlay attributes to discover the corresponding media overlays for EPUB content documents.

Reading systems MUST support playback for XHTML content documents, and MAY support SVG content documents.

Playback MUST start...

This makes it sound like (full/proper) support for MOs is optional, but RSes still need to do all the processing. I don't think that's what's intended. I assume we mean "if you are going to support MOs, then here is what is a requirement for support." Elsewhere, we use language like "If a reading system supports scripting...". I think we should sprinkle such language throughout the MO section, assuming my interpretation is correct.