Convert video to mp3

From here

ffmpeg -i video.mp4 -vn \
       -acodec libmp3lame -ac 2 -ab 160k -ar 48000 \
        audio.mp3

or if you want to use Variable Bitrate Encoding (VBR):

ffmpeg -i video.mp4 -vn \
       -acodec libmp3lame -ac 2 -qscale:a 4 -ar 48000 \
        audio.mp3