Music may be something that you hear, but it does have a visual element. That visual aspect is different for everyone — I see melodies and harmonies as shapes, with a chord laid out as a fragment of its parent-scale. It's asily moved around and re-arranged, but it's always a part of a larger thing.
Other people see sound as colour — it's called "Sound -> colour Synesthesia," and it sounds intense as all get out. I've known a couple of people whose brains worked like this — their musical hearing went beyond perfect pitch into a visual realm. It left them able to hear notes and harmonies in a unique and mystifying (to me) way.
This camera, called "Audible colour," is also a synesthete. It's a musical instrument designed by Hideaki Matsui and Momo Miyazaki that "reads" colours on a spectrum that assigns various colours to various notes. It's a nifty way to generate (and observe) music, and the melodies it creates are soothing and unpredictable.
From Matsui's website:
Audible colour is an audio-visual instrument. Sound is generated based on colour detected by a web cam connected to a computer. Red, green and blue correspond with certain music notes. When the colours are mixed, the resulting secondary colours produce different notes. The size of the colours influences the volume and frequency of the notes played. colour detection and sound generation were created and are controlled using Processing code.
If you've played a game like Flower or Sworcery, where musical tones are generated by touching things in the environment, you've probably seen something similar. The world is made into natural music, and the resulting melodies are just as beautiful as notes on a page.
Audible Color [Hideaki Matsui, thanks G.A.!]