How did a Christmas carol get into an action game? Suzuki thought about using a public domain (i.e. a song you don’t need to pay royalties for) music for The 3rd Birthday, but using a licensed song alone wouldn’t be interesting. Instead he chose to produce a cover.
The Square Enix music team recorded the Joy to the World cover on a cassette tape. Director Hajime Tabata heard the song and thought it was incredibly interesting and decided to put their work into the game. You can listen to the song over the official blog.
A tip of the hat and extra pack of Maeda bullets to Ereek for the tip!