Moonlight Serenade

"Moonlight Serenade" was a song which Jack Harkness played while he was discussing "business" with Rose Tyler, whom he believed to be a Time Agent. (TV: The Empty Child) Jack later played it again for Rose while he was getting her and the Ninth Doctor on board, and then again when she was trying to get the Doctor to learn how to dance. (TV: The Doctor Dances)

Behind the scenes
In the real world, "Moonlight Serenade" would not be released until 1943, two years after this episode is set.