mpv can be used to play Audible's AAX files directly by
passing your activation bytes to mpv via
mpv --demuxer-lavf-o=activation_bytes=12345678 audiobook.aax
Since I always forget passing the extra option, adding it to mpv's config file allows me to skip it. What's even better is we can define an auto profile based on the file extension in the configuration file, so that it is only passed when opening *.aax files, e.g.:
Coupled with resume-playback functionality and chapter support, mpv becomes a great solution to play AAX audiobooks directly without transcoding.