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.