Unfortunately, I can't find any videos, but the Richard Rogers-Lorenz Hart song "Mimi" from Rouben Mamoulian's classic
Love Me Tonight (1932) goes on for maybe seven minutes, and it's sung, at different times, by Maurice Chevalier, C. Aubrey Smith, Charlie Butterworth, Charles Ruggles, Elizabeth Patterson, Ethel Griffes and Blanche Friderici. It's certainly the cutest, sexiest, funniest (yeah, those can all go together), and "epic-est" song I know from a flick.
I also have a soft spot for the finale of
Hair, "Let the Sunshine In". I can't find any appropriate images from the film's ending, but this is a great shot from when the Title Song is sung.