The tools you need are described on blog.joff3.com. There's also the nice youtube-dl package. One thing I noticed: the ffmpeg command there re-encodes the audio which might loose some quality. To just dump the audio off the Flash video, use:

ffmpeg -i somevideo.flv -acodec copy output.mp3

This is for 240px videos. All larger ones use AAC format. Use this:

ffmpeg -i somevideo.flv -acodec copy output.aac
ffmpeg -i somevideo.mp4 -acodec copy output.aac

If you happen to have the latest webm format, use this:

ffmpeg -i somevideo.webm -vn -acodec copy output.ogg

This will just extract the audio track without doing anything to it.

You could also try vixy.net.