From time to time I find it useful to extract audio (MP3 format) from a video (MP4). My camera takes pretty good video, but a recent how-to video project involved editing several videos, pictures and audio together to make sense of all my ramblings.
In this case, I had video with audio I wanted to use and vice versa and wanted to mix them together .. into a final version video using another program called the Openshot Video Editor.
Convert MP4 to MP3 on Ubuntu 14.04 Trusty
There is a program available on Ubuntu youtube-dl, that can download videos from youtube.com or other video platforms. But it stores the file in mp4, since it contains video. But sometimes you do not want the video and only the audio. To extract only the audio part, you can use the pacpl comamnd line tool.
pacpl --to mp3 -v -r- -bitrate 320 targetfile.mp4
Had I checked, this program was already listed in the Ubuntu Software Center so I could have just searched for it and installed it from there just as I did for the Openshot Video Editor program.