A simple script to pull mp3s out of mp4s.

extract_mp3_from_mp4

#!/bin/sh

usage() {
    (
        if [[ $# -ne 0 ]]
        then
            echo $*
            echo
        fi
        echo "USAGE: `basename $0` <input.mp4>"
        echo
    )>&2
    exit 1
}

[[ $# -eq 1 ]] || usage

INPUT=$1
[[ -f $INPUT ]] || usage "Error: Input file not found:" $INPUT

EXTENSION="${INPUT##*.}"
[[ $EXTENSION == "mp4" ]] || usage "Error: Input file must be an mp4:" $INPUT

OUTPUT=`basename $INPUT .mp4`.mp3

ffmpeg -i $INPUT -vn -b:a 192k $OUTPUT