There is relatively untapped power in using music, specifically source music (that which is heard or played or sung by characters in a non-musical), as a cinematic storytelling tool.
I came upon this notion last night after watching Hitchcock's The Man Who Knew Too Much (1956), starring Jimmy Stewart and Doris Day. Day being, at the time, more famous as a singer than an actress, I suppose it seemed only natural to try and incorporate her hit "Que Sera Sera" into the film some how. When it first appears early in the film, sung playfully by Day and her onscreen son, it reads as just a little forced and corny. But later, once the film has built to it's suspenseful climax, Day's character desperately recapitulates the song in an attempt to find her kidnapped child. Plunking percussively at a grand piano, she raggedly belts the tune until her son whistles back an answer.