This is an interesting topic. I have an adult son who has music synesthesia. When he hears music he also sees music in colors for notes. He has always had this and when he was young he thought that was just how it was for everyone. There is also number synesthesia where people see colors as numbers, i.e. a 2 is always orange and will be orange for the rest of their life. You may want to just Google synesthesia, as there are many forms from seeing words to tasting, etc. Sorry I cannot post any links yet. There is also a musical group named Synesthesia, so not to confuse
Synesthesia is viewed by some as having a 6th sense. There is no medical treatment as none is needed. Some feel it is merely an enhancement of their senses. Medically, it appears that possibly when the senses were developing, some got "wired" a bit too close, although not all agree to this theory.
Of course, when he first told me I immediately researched it and found a possible connection to temporal lobe epilepsy, ( which he does not have ) but it does appear the research on this is all over the place, with most only considering it a neurological because it occurs in the brain, but does not appear to cause any medical or psychological deficit, rather the opposite. For my son, he sees it not as a 6th sense, but just part of who he is.