What does the music do for the mood of the film and how are scenes enhanced by the choice of song playing in the scene?

On the subway, when Dito and Mike O’Shea first talk, the dialogue is offered in two versions. One seems to be diogetic (happening in the scene) and one seems to be non-diogetic (happening in voice over. What is your interpretation of both versions. Why is this conversation offered in two different styles.

A third into the movie, the story is interrupted with each central character talking to the camera about who they think they are. How do these testimonials affect the story being told? Do they seem to support or contradict the versions of each character we have known so far?