I was listening to one of my new Sarah Slean CDs the other day. Elijah raced in to the room, stopped for a moment, cocked his head, listened and said, "Thas a guitar music!" Indeed it was: an acoustic guitar in that instance.
Tonight, I had another CD on and I asked Elijah what kind of music was playing. "Thas a pwano music." Right again. Later he said, "Thas a lady singing." Correct again! Then he put two and two together: "Thas a lady singing, pwaying pwano."
There's nothing in the parenting books about the development of children's aural identification skills, so I don't know whether this is normal or not. I'm guessing though, that most two-year-olds probably haven't come into contact with these instruments, let alone can tell what they are just by listening.