Abigail's Song

"Abigail's Song" was a song that Abigail Pettigrew once sang at the Eleventh Doctor's insistence. Because of the perfection of her voice, she was able to use the melody to resonate the fog crystals in the skies above Sardicktown. This, in turn, unlocked the clouds, created snow and allowed an endangered spaceship — carrying a honeymooning Amy Pond and Rory Williams — to land safely on the planet. (TV: A Christmas Carol)

Behind the scenes
The song is not named in narrative. Its name is derived from the soundtrack to A Christmas Carol. Written by Murray Gold and performed by Katherine Jenkins, it prominently featured in the last half of the story.

As with TV: The Gunfighters' "The Ballad of the Last Chance Saloon", it is not just diegetic, but actually vital to the plot.