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.