My youngest likes humming and singing more than his older brother. I might be mistaken, of course, since he's only four years old. It's just that he sometimes sings himself to sleep or sings and sings while waiting. Quite different from his older brother, who'd rather read a book or play one of his gadgets, to pass boredom. The youngest has shown interest in musical instruments, too; like guitars (btw, found one of the best js24p at musician's friend just recently, online). He said he'd like to learn how to play the guitar when he gets older.
My reaction? Am so happy for him! Hopefully, he will still show the same enthusiasm when he gets older. :)