Proposed change to embedded media:

Current text:

When a text element references embedded audio or video, the audio sibling element is OPTIONAL. Reading Systems will intiate playback of the media in the absence of an audio element.

But the audio sibling is optional anyway. So how about instead:

When a text element references embedded audio or video, Reading Systems will intiate playback of the media in the absence of an audio element sibling. Therefore, EPUB Creators should carefully examine any overlapping timed media situations and deal with them at the production stage, as Reading Systems handling of many aspects of simultaneous timed media is implementation-specific.

EPUB Creators SHOULD avoid using scripts to control playback of referenced embedded EPUB Content Document media, as this might conflict with Media Overlays playback behavior.

Ah, good catch, @marisademeglio, I did not realize that the text needed changes in that section as well! I have taken over your text.

I have also added an item to the changes' section. Although the change may be considered as editorial only, I think it is worth drawing attention to this clarification.

In addition to the inline comment, I would remove the paragraph at line 8166:

EPUB Creators should carefully examine any overlapping audio situations and deal with
them at the production stage, as Reading Systems handling of simultaneous volume levels is optional.

Because it is superseded by the statement about simultaneous timed media around line 8157 (I just replaced "volume" with "many aspects" since there are more things than volume that could get hairy).

Agree. I have removed it.

Looking at what I think is the latest, I see an inconsistency. We have made the audio element optional but this section still says (and I think I suggested part of this, now I am un-suggesting it):

A Media Overlay par element MAY omit an audio element when its text element references text or an image. In these cases, Reading Systems are expected to render the referenced fragment via TTS, so EPUB Creators MUST ensure the fragment is appropriate for TTS rendering (e.g., contains a textual EPUB Content Document element or has a text fallback).

So how about, to align with our other wording changes:

When a Media Overlay par element omits an audio element, its text element may be rendered in Reading Systems via TTS. If the text fragment is not appropriate for TTS rendering (e.g., is not a text element and/or has no text fallback), this may produce unexpected results.

@marisademeglio see #2008