“mplayer -dumpaudio GummyBearSong.avi -dumpfile gummybear.mp3”

Where:

“GummyBearSong.avi” is the source video, in the example above it was 30 MB
“gummybear.mp3” is the output audio file (in mp3 format), was about 3 MB in size

Another method that worked flawlessly with an MP4 file is using ffmpeg:

ffmpeg -i SOURCE.mp4 -vn -acodec libmp3lame OUTPUT.mp3