The sound implementation in Flash has been greatly improved with Flash CS3:
http://livedocs.adobe.com/flash/9.0/main/00000283.html
Here is a great example of a simple sound visualizer:
http://livedocs.adobe.com/flash/9.0/main/00000296.html
Note the following limitations:
Because sound data from a microphone or from RTMP streams do not pass through the global SoundMixer object, the SoundMixer.computeSpectrum() method will not return data from those sources.
