You can easily extract audio from video files such as avi, mpg, even flv into mp3 uses ffmpeg. You can even record online stream into mp3, such as stream from radio cast.
The audio component can be extracted to an mp3 using floowing command (from the command line Terminal). (This will work for any type of video file, not just Flash.)
ffmpeg -i video.flv -ab 160k -ac 2 -ar 44100 -vn audio.mp3-i indicates the input
-ab indicates the bit rate (in this example 160kb/sec)
-vn means no video ouput
-ac 2 means 2 channels
-ar 44100 indicates the sampling frequency.
See FFMPEG docs for more info