How to save audio of video as MP3

There are times when I want to save the audio stream of a video file as a MP3 file. I tried using Sound Converter for this, but it would fail while trying to install plugins. But using FFMPEG worked fine!

To save as a variable bit rate MP3:

$ ffmpeg -i in.mp4 -vn -acodec libmp3lame -ac 2 -qscale:a 4 -ar 48000 out.mp3

Tried with: FFMPEG 2.7.6 and Ubuntu 14.04