Files downloaded with the Audible Download Manager are stored with the .aax file extension, but the information I can find about this format seem to vary.

Regarding this source, the files are stored in the M4B container (MPEG-4 Part 14), also a quick look into the file header of the .aax files reveals aax M4B mp42isom.

Furthermore, regarding this source, the audio is encoded with the ACELP codec.

If I would decrypt the .aax files DRM with my Audible authentication code without converting the audio, which format and codec would the audio be in?

(Also I'm not sure if this belongs here or in "Sound Design".)

  • 2
    $\begingroup$ Audible books are in m4b format. If you have the authentication code then just use FFmpeg (a relatively new version of it) and convert it to m4b format: ffmpeg -activation_bytes KEY -i book.aax -c copy book.m4b $\endgroup$
    – jojeck
    Oct 7, 2020 at 9:14
  • $\begingroup$ and ACELP isn't really "a" codec, but the math basic for a whole range of codecs; some of them very proprietary, some of them MPEG4 standards, some really obscure, some in wide use... $\endgroup$ Oct 7, 2020 at 9:28
  • $\begingroup$ @MarcusMüller would you formulate the answer? $\endgroup$
    – jojeck
    Oct 7, 2020 at 9:53
  • $\begingroup$ nah, it's not an answer – I don't even know what's inside the encrypted file, I was just raising caution about the cited source. $\endgroup$ Oct 7, 2020 at 10:06
  • $\begingroup$ I would like to add that I'm not too familiar with codecs, containers and formats and the differences between them. I assumed ACELP is a codec because in the second source it said "they contain data encoded with either the MP3 or ACELP codecs". $\endgroup$ Oct 7, 2020 at 22:58

1 Answer 1


As this post over at the apple stackexchange says, .aax files from Audible are indeed encrypted m4b files, which means that if I would decode an .aax file without converting it, I would get a .m4b file.

This is possible with the following command:

ffmpeg.exe -activation_bytes XXXXXX -i input.aax -c copy output.m4b

The -c copy ensures that the data will not be converted.


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