In my view, audiobooks are a complete waste of time for retaining information unless you are highly engaged and not distracted.
The droves of multitaskers that "mind hack / life hack" themselves by listening to audiobooks at 2x+ speeds while "getting shit done" are deluding themselves and descending into stupidity.
Read, god dammit! Read! You have to read deeply and deliberately.
Seriously, audio books are a good supplement but not a replacement. You cannot be educated "on the go" with audio books. Anything worth learning will demand your full attention.
That's my opinion.
When did they add the sync feature? I asked the audible support about sync two months ago and they said that the notes are only stored on the device you are using. Indeed i couldn't see it anywhere else and when i switched phones the notes wouldn't sync to the new audible installation.
It must have been recently then, I've just looked at my notes in the web player that I had added in the Android app. If you still happen to have your old phone, you may be able to sync them now.