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".)

• 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
– jojek
Oct 7 '20 at 9:14
• 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... Oct 7 '20 at 9:28
• @MarcusMüller would you formulate the answer?
– jojek
Oct 7 '20 at 9:53
• 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. Oct 7 '20 at 10:06
• 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". Oct 7 '20 at 22:58

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

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