@aaron
Legally it doesn't matter if, where or whether the books are still on sale.
It is still copyright infringement -- namely of the exclusive reproduction right -- to make them available without the permission of the copyright holder.
Also note that the copyright is separate for each work -- the spoken narration of a book in the public domain enjoys its own copyright protection independent from the
underlying literary work.
So if you download and make available a literary work from Project Gutenberg, you are in the clear provided that the work is no longer under copyright in your country, but the publisher of the audiobook may still claim copyright in the audio production, and it's therefore not legal to make the audiobook available without his permission.
An audiobook originating from a cd or tape casette is certainly still under copyright protection, and it's no legal defense that it has gone out of print.
And that's the reason why copyright sucks.
On a technical note, BT Sync is not anonymous, everyone participating in downloading is also making pieces of the folder available to others in the swarm.
m