Singing. I suppose he may have been singing even sooner, but now he is actually matching pitch. His favorite song is the ABC's. He gets A,B,C,D,E, O,P, and T,U,V, and Z accurately. He really enjoys watching me fingerspell along, and moves his fingers and hands along.
I shouldn't be surprised that the alphabet song is his favorite. His favorite toys are the letter magnets and the LeapFrog Word Whammer , Fridge Phonics set.