The only time I've ever seen anything about it was on a program, where they showed a nifty little demonstration of what they thought synaesthetes see or hear. It was done by a study in progress of a dozen people who had varying degrees of synaesthesia and the demonstration was put together from information gathered from them. When studied separately, each person had similar experiences......as far as tones in relationship to colour and colour from tones. As less than 4% of the population has it, I imagine it's not the easiest thing to make any conclusions about.
All I know, is I'd sure like to know more.