Well, what is synaesthesia? Everybody knows the word ‘anesthesia’ which means no sensation.

Synaesthesia means joined sensation, where two or more of the senses are hooked together, so that my voice for example is not only something that they hear but also something that they see or taste or touch.

The most common form of synaesthesia is colored letters and numbers. That is, joining color to integers. That accounts for about two-thirds of cases. The next big group would be sight and sound synaesthesia, or what is called colored hearing. In this, voices, music, environmental sounds will make people see colored photisms—these are shapes that arise, they change and metamorphose a little bit and fade away. Think of it as a little bit like fireworks. So they have a location and space they move around. And they enjoy it very much. There is almost a eureka sensation with this. They can’t imagine what listening to music is like for the rest of us. Of how do we remember people’s names or phone numbers if there is no color there to help us?

more here.