The files with .aax extension are audiobooks developed by Audible; an American online podcast service and audiobook owned by Amazon.com Inc. These files may be used by the Audiblekids, Audible.com and iTune store. The AAX files are digital multimedia audiobooks which may include features such as, graphics, videos, bookmarks and a video timeline; making them more useful and suitable choice for Picture Books and Graphic Novels for kids. The AAX files are considered as enhanced version of AA files.
AAX file format
AAX is an Audiobook file format, which is a M4B file with high quality due to its variable-bitrate and encrypted with DRM (Digital Rights Management) encryption algorithm. Majority of ebooks are encoded at 64 kbit/s, 22.050 kHz, stereo, some are as low as 32k, mono and radio plays are usually encoded at 128kbit/s and 44.1 kHz. The audio of AAX file format is encoded in AAC format (variable quality).
AAX file format specifications
The detail about AAX file format quality specs available at audible is shown below:
Bitrate: 32 - 128 kbit/s
Sample rate: 22.050 - 44.10 kHz
Bit Depth: Unknown
Channel: Mono or stereo
Container: MPEG-4 Part 14
Quality description: AAC sound
Digital rights management (DRM)
Audible file formats encapsulate the encoded sounds but they also include the prevention for un-authorized playbacks by means of an Audible username and password, which can be used by-default on up to four computers and three smartphones at a time. Schools and libraries may be offered different types of licenses. The DRM is responsible to control:
The un-authorized user access
Burn a limited number of CDs for unrestricted playback
Enable Audible content for limited number of devices
But you can still find many other software tools which can easily bypass the DRM control of Audible. Audible is now offering DRM-free titles for content writers. The FFmpeg 2.8.1 or greater versions can play .aax file formats natively.