AA - Audible audio format is used for digital audiobooks released by Audible. It is essentially DRM protected .mp3 file, which means you will have to use some kind of DRM removing tool to get the original .mp3 back, which is why a lot of users are looking for aa to mp3 conversion.
You can find some utilities for this, but be advised that such operation is most likely breech of Audible user agreement.Open AA file Open MP3 file
A DRM remover and media converter
A program for removing DRM protection from multimedia files