According to their FAQ section:
Is Audiobooks.com mobile friendly?
It sure is! You can stream or
download your favorite books from Audiobooks.com on your iPhone or
iPad (iOS4.0 or higher), or Android smartphone (running version 2.3 or
higher), via our native apps.
Thanks to our HTML5 web app, you can
also listen to audio books using ...
It really depends on the type of content you are producing and how many sections there are (and what they are about).
I've heard lots of readers just give a longer pause between sections, and that worked well.
On the other hand, flashbacks are already a unique beast and we are already accustomed to hearing or seeing some kind of cue. The standard I'd use ...
I don't think they are any rules, but the important thing is that the effects sound professional and not too distracting. The problem is that if you commit to one sound effect, you are implicitly agreeing to produce other sound effects as well -- which may or may not be easy.
Not that I'm into sci fi but when teaching overseas I happened to hear a Star ...