Convert `.mp4` to `.mp3` using ffmpeg

To convert `.mp4` to `.mp3` using ffmpeg, do as follows:

ffmpeg -i moon_20160905.mp4 -vn -acodec libmp3lame moon_20160905.mp3

Reference:
http://stackoverflow.com/questions/3032280/how-to-convert-mp4-to-mp3-in-java