How to extract the audio from an mp4 video to a mp3 file?

You can use ffmpeg:

ffmpeg -i /PATH/TO/INPUT.mp4 /PATH/TO/OUTPUT.mp3

If it is not installed, you can install it with:

sudo apt-get install ffmpeg

You can find a help page on using ffmpeg here, or by running ffmpeg --help or man ffmpeg. On some versions of ubuntu it is sometimes called avconv, which is largely similar in functionality.

For the above commands, you use the Terminal, which can be accessed by opening it from the dash/launcher, or by pressing Ctrl+Alt+T.

If you don't want to use the terminal and you don't have any real demands to do the job, you can always use VLC player to do this, which is probably installed in your distribution.

1. Open VLC player.
2. From the menu bar, please select "Media" and then "Convert/Save".
3. Click "Add" and select your file.
4. Click "Convert/Save".
5. You should then select "Audio - MP3", enter a name in the "Destination File" and then click "Start".

Your file should converted to MP3 format in no time.

I think that the most detailed answer with several options can be found here:

How can I convert audio from MP4 or FLV video files to mp3?