for f in *.mp4; do ffmpeg -i "$f" "`echo $f | sed -e "s/.mp4/.mp3/"`"; done
$ ls
a.mp4 b.mp4
$ for f in *.mp4; do ffmpeg -i "$f" "`echo $f | sed -e "s/.mp4/.mp3/"`"; done
$ ls
a.mp4 a.mp3 b.mp4 b.mp3
$ chmod +x mp4tomp3.sh
$ sudo cp mp4tomp3.sh /usr/local/bin/