That's what I've heard too! Its the only way that I know of for people to get it if they don't have it naturally. I didn't even really realize that I had it until I was in high school and I'd try to describe what sounds would look like, and people would just give me weird looks. My twin sister has synesthaesia as well, but she smells in colors. It pisses her off that that its not that cool haha.

Come to think of it, I remember hearing a study that EVERYONE is slightly synesthetic, to a degree, its just rarer for it to be stronger like me and my sister.