I can't be the only one that reads those two examples very differently.

In a first example, the second "occasionally" is stressed, when in second example it feels almost whispered.

Interesting, I read both as emphasized. To me, the parens make the voice clear but the em dashes make it read like a typo.

I read the parenthesis as a secondary thought that is there if you want to take in additional context but where the main message can be read just as well without, whereas the em block is a deliberate interruption to call attention to context necessary to understand the main message.

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.