Try LSD. From what I've heard, it makes you see smells, hear colors, taste sounds, etc,
Actually the true definition of synaesthesia, according to the dictionary, is a sensation produced in one modality when a stimulus is applied to another modality. In other words it is not limited to seeing color when hearing music, though that is how we most often think of it.
Proust's "In Search of Lost Time" featured this condition in what is known as involuntary memory. A character for instance would feel the fabric of the drapes and have experiences from the past come back.
Everyone really has this ability, some more latent then others, but I believe it can be developed by concentrating on senses that are aroused when playing or listening to music, and trying to identify what
they identify. Images you get, memories you have, or even just unidentified sensation are all fodder for a deeper level of association and subjective identification.
I don't "see" colors when I hear music, yet sometimes, I have the inexplicable feeling that a certain passage and a certain color "feel" the same, and they are identified in my mind as one thing. It isn't just colors though; a certain musty book smell makes me hear certain works of Bach, which I played from an old, moldy book bought at some second-hand shop. The senses cross, and synthesize, and a new level of characterization is possible.
I think, actually that we should strive to find these for the works that we play. We should strive to make the music we play as much internal as possible, and to that extent, it requires that we integrate the music - the new sensations - with our enormous, never-ending repertory of stored sensation. Sometimes this work seems to be done for us, but other times we have to search, to find the personal meaning of a passage or piece. Neuhaus calls this the "artistic image."
Cortot also required his students to come up not only with theoretical analyses, but also poetic images. In some of his writings, he describes his own, and they are unforgettable. For instance in a rolled-chord passage in Franck's Prelude, Chorale & Fugue he describes the rolled chords as "flames alighting the horizon." And I could truly see it in connection with the music. That is synesthesia! Or when Thomas Ades describes his music (Traced Overhead, I think) as "illuminated from within." Or when Wagner incorporates the smallest detail of movement, down to facial expression, of his characters into the orchestral fabric.
Perhaps getting to the root of synesthsia, is getting to the very root of music itself. Music as an empty vessel, ready to be filled with all of our free associations, ready to take on whatever meaning we can confidently ascribe to it, even none at all.
Walter Ramsey