The musical score for Once Upon A Time In America was composed, arranged, and conducted by Ennio Morricone. Morricone scored all of Leone's other films such as Once Upon A Time In The West and The Good, The Bad, and The Ugly. He has written scores for over 300 feature films. Among the most memorable are The Battle of Algiers, Novecento, Days of Heaven, The Mission, The Untouchables, Cinema Paradiso, State of Grace, and Malena. Much of the score for Once Upon A Time In America was composed before the film was shot and was even played on set during shooting.

Liner notes for the 1998 re-release of the soundtrack on Rykodisc CD by Jon Burlingame (PDF)


From Wikipedia:

Besides the original music, the movie also uses several pieces of 'found music', including:

'God Bless America' (written by Irving Berlin, performed by Kate Smith - 1943) - Plays over the opening credits from a radio in Eve's bedroom.

'Yesterday' (written by John Lennon and Paul McCartney - 1965) - An instrumental version of this piece plays when Noodles' first returns to New York in 1968, examining himself in a train station mirror. A version of the song also plays briefly during the dialogue scene between Noodles and "Bailey" towards the end of the film.

'Amapola' (written by Joseph M. Le Calle - 1923) - Actually originally an opera piece, several instrumental versions of this song were played during the film; a jazzy version which played on the gramophone danced to by young Deborah in 1922; a similar version played by Fat Moe's jazz band in the speakeasy in 1932; and a string version, during Noodles' date with Deborah. Both versions are available on the soundtrack.

'The Thieving Magpie' (Overture) (Gioacchino Rossini - 1817) - Used during the baby-switching scene in the hospital.

'Summertime' (George Gershwin - 1935) - Played by a jazz band during the beach scene after the beachgoers receive word of Prohibition's repeal.

'Night and Day' (written and sung by Cole Porter - 1932) - Plays during Secretary Bailey's party in 1968.

Unfortunately these tracks, except for 'Amapola', are not included on the released soundtrack album.