Q. How do I cite a song in MLA style

Sound recordings are cited according to the desired emphasis. In other words, the first item cited could be the composer, conductor, ensemble or performer, depending on what you are discussing in your paper.

The Basics

Person (composer, conductor, ensemble or performer, depending on what you are discussing in your paper). Title of recording (or title of the works included). Artist (when different from the first person listed in citation). Manufacturer, year of issue n.d if date unknown). Medium.  

Example Song
A Jolly Christmas. Capitol, 1957. CD.

Example MP3
Sinatra, Frank. "White Christmas" By Irving Berlin. A Jolly Christmas. Capitol, 1957. MP3 file.