Examples that qualify: SINGIN' IN THE RAIN qualifies because all of its songs are NOT written for the film (they were Freed songs written way-back-when).
Brown & Freed wrote one original song for the film, "Make 'Em Laugh", even though it was virtually a remake of Porter's "Be A Clown" (both musically and lyrically)
I've often thought of Alfred Newman as one of the foremost promoters of the great American song repertoire, not just for his work on film musicals, but his use of popular music in dramatic films. A few examples: THE GRAPES OF WRATH WILSON A TREE GROWS IN BROOKLYN
As to David Raksin's SHOCKING credit, I believe he also arranged, without screen credit, "On The Atcheson, Topeka and the Santa Fe" in THE HARVEY GIRLS, (w/vocal arrangement, I thnk, from Kay Thompspn).
The 1953 version of THE JAZZ SINGER, starring Danny Thomas, includes the following songs:
"Living the Life I Love," "I Hear the Music Now," "What Are New Yorkers Made Of" and "Lu Lulla Lu (Hush-a-Bye)," music and lyrics by Sammy Fain and Jerry Seelen "Lover," music and lyrics by Richard Rodgers and Lorenz Hart "Just One of Those Things," music and lyrics by Cole Porter "I'll String Along with You," music and lyrics by Al Dubin and Harry Warren "The Birth of the Blues," music and lyrics by B. G. DeSylva, Lew Brown and Ray Henderson "This Is a Very Special Day," music and lyrics by Peggy Lee