To convert a MP4 file to a MP3 using ffmpeg

ffmpeg -i video.mp4 musiic.mp3

or 

ffmpeg -i video.mp4 -b:a 192K -vn music.mp3

for variable bit rate

ffmpeg -i k.mp4 -q:a 0 -map a k.mp3

Useful Links
https://superuser.com/questions/332347/how-can-i-convert-mp4-video-to-mp3-audio-with-ffmpeg
https://trac.ffmpeg.org/wiki/Encode/MP3
 

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