This is little computeSpectrum Actionscript 3 example for Flash
There is huge improvement in AS3 with Sound and SoundChannel class.
basiclly computeSpectrum is simple and there are few good examples already available
mine uses some BitmapData filtering examples too
if you download fla just put song.mp3 in same folder
UPDATE : two more examples, different graphics but same algo
EDIT: Links are corrected, sorry for keep you waiting. Thanks!