Hark the Herald Angels Sing

"" was a Christmas song.

When the Second Doctor and Jamie McCrimmon were imprisoned in Trenton with time travellers Stephanie and Edward, they heard their Hessian guards singing "Hark the Herald Angels Sing" in German. (PROSE: The Revolutionaries)

Edith Thompson sang the song on Christmas Eve in 1906, but she did not know all the words, instead humming through the missing parts. (AUDIO: The Chimes of Midnight)

Behind the scenes
The song is heard being performed by a choir on the soundtrack of the episode The Husbands of River Song near the start of the episode. There is no clear evidence that this song is actually being performed diegetically on the planet Mendorax Dellora.