AA (Audible Audio) is an audio file format developed by Audible.com used for storing and distributing audio books sold on their website.
The books on Audible.com can be downloaded in different versions and sound qualities, and the AA format is used for the lower quality audiobook versions, contrary to AAX, which is the higher quality alternative to the AA format.
In its essence, the audio data stored in AA files is encoded at low bit rates using either MP3 or AAC encoding. However, what makes AA files different from standard MP3 and AAC files is that they may also contain meta data (such as chapter markers, bookmarks etc.) and that they also utilize copy-protection through a proprietary digital rights management (DRM) technology.
To further ensure that copying AA files is as hard as possible, Adubile.com keeps the exact AA file format specification closed to the public, making it extremely hard for third-party developers to break the DRM encryption and invent tools that allow you to grab the necessary audio data and convert it to a popular open audio format like MP3 for example. Therefore playback of AA files is heavily limited to a narrow set of Audible-ready devices such as the Amazon Kindle or the Apple iPod.
Nevertheless, there are a few clever tricks, such as playing the AA file through Windows Media Player and capturing the stereo output of your sound interface with Audacity in order to grab the audio data and export it to a more convenient audio format that can be played on any portable media player.