Convert videos and extract audio with ffmpeg

Extract mp3 from avi:

ffmpeg -i source_video.avi -vn -ar 44100 -ac 2 -ab 192 -f mp3 sound.mp3

Convert flv to avi:

ffmpeg -i source_video.flv -sameq final_video.avi

More information: http://www.webupd8.org/2009/08/ffmpeg-cheat-sheet-19-best-practices.html