Oliver Sacks ...
... wrote a small piece here on Synethesia. and on this page there are wonderful interviews with other people also...
http://performancetoday.publicradio.org/features/2009/02/synesthesia/
After reading this I became more aware of what it was...but less capable of finding a way to make a photo...
Very few people have the condition. And when they do it is not the same for any two people..
In other words no two people necessarily have the same things they hear, or they see...and associate.
Wikipedia has a rather short article on the neural basis of synesthesia:
http://en.wikipedia.org/wiki/Neural_basis_of_synesthesia
where two leading theories are briefly described on the cause of synesthesia. One attributes it to the "cross-talk" of adjacent regions in the brain that are specialized for different sensory processing. The other attributes it to a weak or the absence of the inhibition of the feedback path of sensory signals.
Clearly the experience of synesthesia is natural or innate to those who are capable of experiencing it and is not built on learned experience. It is different from the experience for the rest who may have synthesized sensation through association learned from life experience, processed by the 'association area' of the brain.
So for the rest of us, we are hopeless in attempting to have the innate synesthesia experience, let alone of producing a photo reflecting that. I have sensed in this thread some "desperation" in trying to create a photo meeting Kathryn's challenge :-)
But then there is hope - in the same wiki article, it mentions that some psychedelic drug users report synesthesia-like experiences, although that is absolutely not something I would suggest or recommend our serious but "desperate" photographers to try :-)
So I think I would settle the second best option for the associated synthesized experience. Even that is hard. And I still don't have a clue :-)
Nevertheless, this is fun, thanks to Kathryn's original topic...