<p>So how does someone create a music visualizer? I've looked on Google but I haven't really found anything that talks about the actual programming; mostly just links to plug-ins or visualizing applications.</p>

<p>I use iTunes but I realize that I need Xcode to program for that (I'm currently deployed in Iraq and can't download that large of a file). So right now I'm just interested in learning "the theory" behind it, like processing the frequencies and whatever else is required.</p>

Typically, you take a certain amount of the audio data, run a frequency analysis over it, and use that data to modify some graphic that's being displayed over and over. The obvious way to do the frequency analysis is with an
FFT
, but simple
tone detection
can work just as well, with a lower lower computational overhead.
So, for example, you write a routine that continually draws a series of shapes arranged in a circle. You then use the dominant frequencies to determine the color of the circles, and use the volume to set the size.