The Audio Visualizer is the little box on the bottom left of the screen whilst playing. When there is no music playing, it stays still, and when music is playing, it adjusts to the volume/beat/etc. of the sound.
How does that work? Is there a video file associated with each track that plays when the track plays, or is there some way that the game adjusts the audio bars to what is playing? If so, would it play differently to a custom track?