Quick reference: Audio and Video Commands in Linux (Ubuntu)

Extract audio from a movie and save as mp3
ffmpeg -i input.avi -vn -acodec libmp3lame -ar 22050 -ab 256k -ac 2 output.mp3

Extract to wav file:
ffmpeg -i input.avi -vn -acodec pcm_s16le -ac 2 output.wav

More info:
http://howto-pages.org/ffmpeg/