The protocols are pretty much set in stone, revealing just as much of an interface between Quartz Composer and iTunes as needed to make visualizations of the music playing and to display the track information.
http://www.qtzlcodex.com/?q=music_visualizer_protocol has a good article about the Music Visualizer protocol, they also have a load of example compositions that you could well look into. To quote from their site
//However, like most of Apple's Quartz Composer material, there is a glaring lack of documentation for users and developers who want to make use of this interesting technology.//
wowlab.net also have a lot of really good example visualizers that make for great study examples. Quartz Composer's own template contains all that you need and can be adapted to a range of purposes
