You should look closer at the actual matrix visualizations, and you'll see there is no z axis aspect to it. It is a stream of symbols flowing down, with an occasional bright "drip" trail representing something quite unusual. So my suggestion is to use the x axis to represent frequency, and have drips occur based on frequency and the speed of these drips determined by say bpm and the occurence of a drip affected by a peak in the frequency. That way it's visually correlated to the audio.

If a matrix is done to represent the mp3 bitstream then it won't be correlated to the audio and will not be as cool. imho.

Calvin