AAX is a audio file format used by Audible company, a subsidiary of Amazon since 2008. AAX is based on variable-bitrate AAC format with added encryption. Encrypted AAX file are used for distributing audiobooks.

AAX format supports various encodings. It can store 32 kbit/s mono tracks at 22,050 kHz as well as 128-bit stereo at 44,10 kHz. AAX files use MPEG-4 part 14 containers and single track takes 28,8 MB/hour of recording.