Mplayer is not only a video/audio-player but also a tool to extract audio from videofile, convert files etc.
Dump audio from Flash video file
Many Flash videofiles contain encapsulated mp3 audio file. You can check the format with VLC. Run flv-file with VLC, open Tools and Codec information. If audio codec is mp3 then all you have to do is
mplayer -dumpaudio original_video_file.flv -dumpfile audiofile.mp3
You can use mplayer to record (save) video and audio streams. You have to know the address of the stream which is not the same as the address of the web page where you found it. However, you can use Firefox add-on UnPlug to find out this necessary information. After this you can save the stream with
mplayer -dumpstream http://url-address-of-stream