Convert Mp4 to Mp3 in Linux Terminal

First things first, install the required software packages.

sudo apt-get install ffmpeg && sudo apt-get install libavcodec-extra-53

Then use it. For FFmpeg with Constant Bitrate Encoding (CBR)

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

If it says ffmpeg is depricated use this command instead.

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

The meaning of arguments

argv	meaning
-i	input file name
-vn	disable video recording
-acodec	force audio codec to libmp3lame
-ac	set the number of audio channels
-ar	set the audio sampling frequency

I hope this helps.