Comment by lesostep

It's really hard to find style guides on reading specifically.

But what I see from trawling the posts made by narrators, a lot of them do mention that em-dashes are read as interruptions with sudden tonal shift, while cursive implies a non-interuptive tonal shift.

That said, apparently, most audiobook authors actually supplement their own guidance on how to read their books. I haven't found any universal guide yet.