I have synesthesia. In my opinion it's like listening to music: the music is sound AND emotion at once, even though sound and emotion are two totally different things. They morph together and become one. That's what synesthesia is like... To me, anyway.
I have sight-sound synesthesia. It is a a recently discovered form of synesthesia. I can hear motion. For example, if I watch a silent video, I can hear sounds that go with motion. It's hard to explain
In a TV program I heard it explained like this: it's basically a "wrongly wired" brain what causes it. Different sections in the brain are tasked with processing different sensations, like hearing, view, smelling and a lot of other ones. Activity in a certain section triggers reactions in another using pathways designed for that purpose, if the sensations are related that is normal. But in some people those pathways got somehow mixed up, and a acoustic signal triggers a reaction in the visual cortex, for example. I had that when I was younger, too. A loud noise would make me see kind of a flash, like a lightning when you have your eyes closed. But over the years that disappeared. Some people have extreme forms of synesthesia though. On TV they showed a guy to whom words have a taste, or a woman who hears in different colors. Really weird stuff... but our brains are weird by nature.