Moonlight Serenade

"Moonlight Serenade" was a song that Jack Harkness played while he was discussing 'business' with Rose Tyler when he believed she was a Time Agent. Jack later played it for Rose while he was getting her and The Doctor on board, and then again when she was trying to get The Doctor to learn how to dance.

Behind the Scenes
This is yet another example of a difference between the real world and the DWU: in the real world, "Moonlight Serenade" would not be released until 1943, two years after this episode is set.