Ed. note: Sorry this is more like a blog post.
Having recorded the Molin Upper Elementary 5th Grade Christmas Concert using my video camera, I wanted to extract just the audio portion so that I could listen to the music all by itself without having to play the video. Besides, I could send the audio portion much more easily than I could send the 948 MB video file through email. A lossless .ogg file would be only 21 MB and 14 MB in lossy mp3 format.
The Video Editing article gives some examples for using mencoder and ffmpeg, but in this case I found using ffmpeg on the command line was the easiest tool after some initial trial and error. I was surprised to find out that Avidemux didn't just do what I wanted. With Avidemux, I tried 'save audio' which produced a relatively large file, that didn't play in anything. I ran ffmpeg -i on it, and it reported "unknown format"
# try mplayer, but it did not work
mplayer -dumpaudio VID00001.AVI -dumpfile molin-5th-grade-xmas-concert.mp3
## Try instead to use ffmpeg
# tell me information about my source file
ffmpeg -i VID00001.AVI
# copy the audio codec; but name the output .mp3 (of course this will not work)
ffmpeg -i VID00001.AVI -acodec copy molin-5th-grade-xmas-concert.mp3
# copy the audio codec; disable video; but name the output .mp3 (of course this will not work)
ffmpeg -i VID00001.AVI -vn -acodec copy molin-5th-grade-xmas-concert.mp3
# disable video; use mp3 codec (that is not the name of a valid codec)
ffmpeg -i VID00001.AVI -vn -acodec mp3 molin-5th-grade-xmas-concert.mp3
# let ffmpeg figure out what I want (WORKED)
ffmpeg -i VID00001.AVI -vn molin-5th-grade-xmas-concert.mp3
For creating the .ogg file, I went with the easier GUI approach and used SoundConverter