Pack up Your Troubles...

"Pack up Your Troubles..." was a song known to Captain Jack Harkness. Its chorus contained the lyrics "smile, smile, smile". Jack was singing a florid rendition of the song while alone in the TARDIS console room when a distress call was picked up originating from the Novrosk Peninsula in Russia, and Jack didn't notice the warning beeps until he was halfway through the next chorus. (PROSE: The Deviant Strain)

Behind the scenes
In the real world, the song's full title is "Pack Up Your Troubles in Your Old Kit-Bag".