convert Audible AAX file to MP4

aax2mp4.sh

#!/usr/bin/env bash

[ $# -ne 1 ] && { echo >&2 "Usage: `basename $0` audiobook.aax"; exit 1; }
type ffmpeg > /dev/null 2>&1 || { echo >&2 "Error: ffmpeg required"; exit 1; }

SRC="$1"
ACTIVATION_BYTES='' # can be obtained by audible-activator <https://github.com/inAudible-NG/audible-activator>
[ "$ACTIVATION_BYTES" = '' ] && read -p '4 byte activation secret (e.g. 1CEB00DA): ' ACTIVATION_BYTES

ffmpeg -activation_bytes "$ACTIVATION_BYTES" -i "$SRC" -vn -c:a copy "${SRC%.aax}.mp4"