According to the official documents, the music score was composed by Chaplin himself, and arranged with the assistance of Alfred Newman. The romance theme was later given lyrics, and became the pop standard "Smile", first recorded by Nat King Cole and later covered by such artists as Sammy Davis Jr., Dean Martin, Tony Bennett, Trini Lopez, Eric Clapton, Barbra Streisand, Diana Ross, Michael Bublé, Petula Clark, Liberace, Judy Garland, Madeleine Peyroux, Plácido Domingo and Dionne Warwick, Michael Jackson and Robert Downey, Jr. (included on the soundtrack for the film Chaplin).
Modern Times was the first film where Chaplin's voice is heard as he performs Léo Daniderff's comical song Je cherche après Titine. Chaplin's version is also known as The Nonsense Song, as his character sings it in gibberish. The lyrics are nonsensical but appear to contain words from French and Italian; the use of deliberately half-intelligible wording for comic effect points the way towards Adenoid Hynkel's speeches in The Great Dictator.
According to film composer David Raksin, he wrote the music as a young man wanting to make a name for himself. Chaplin would sit, often in the washroom, humming tunes and telling Raksin to "take this down". Raksin's job was to turn the humming into a score and create timings and synchronization that fit the situations. Chaplin was a violinist and had some musical knowledge, but he was not an orchestrator and was unfamiliar with synchronization. Raksin later created scores for such films as Laura and The Day After.