ffmpeg -i my_video_file.avi extracted_audio.mp3
Well, in the linux world, it's a pity if you don't give an alternate way to do stuff. So here it is:
mplayer -dumpaudio my_video_file.avi -dumpfile extracted_audio.mp3
This technique can be used to extract audio from AVI, MPG and FLV files.
Photo by Peter Fuchs