My favorite 9-year old sings to herself all the time. Generally, I get to witness it when we're on the phone and playing computer games at the same time. She usually keeps up a stream of songs and chatter. Phenom tells me it's an "only child thing."
She seems to have several songs she sticks to for her chatter. For a while, it's been Sound of Music and Jingle Bells. Lately, it's been the theme song for Indiana Jones. Although, I suspect her frame of reference for Indiana Jones is a video game and not the movie series.
I'm thinking I might need to introduce her to the Star Wars movies. One, so she'll have a new theme song, and two, to set her on a life time of true geekiness.