I ported an old Processing program that plays back MIDI files and visualizes them. Not particularly unique or aesthetic, just an example to build on top.
I used this midi file while tweaking the code.
In my system I can hear the music while I see the visuals. I don’t know if something is required to make the sound work in other OSes.
The source code of the program:
One should be able to find thousands of songs in .mid format. They were popular in early Internet times as a way to include lightweight song files appreciated mostly by the authors of web sites. It can be an interesting file format if we are looking for repetitive (rhythmical) but evolving data sources.