In some movies we can't tell whether the sound we're hearing is part of the fictional world: usually the surprise will be that it is, that music that we think is only for us turns out to be coming from a radio or a performer we hadn't seen. And sometimes -- maybe most famously in the 1946 version of The Killers -- there's an interplay between the diagetic and non-diagetic music, so that the Green Cat piano-player's performance of Flight of the Bumblebee strikingly interacts with the Killers-leitmotif when they enter the bar.
But (as I noticed the other day) the opposite sort of thing happens in City Lights. As with the three-decades of silent films it mimics and closes, there's plenty of sound in the world of the film that we in the real world don't hear. Sound movies invert the relationship of sound to world that we find in silent movies. Not quite -- while in silent movies all we can hear is non-diagetic sound, it is true that the silent characters don't hear that sound at all. All they hear is diagetic sound -- to use a double solecism because this is sound we don't hear.
City Lights is the real inverse, though. We and the boxers both hear the bell; we and the robbers both hear the gun go off. And of course we hear all the non-diagetic music that we learned to listen for from silents, if not from opera and ballet, which the characters don't.
But the Blind Girl: what she hears we don't. She plays a record on the phonograph, but the soundtrack music doesn't change. More significantly, in the intensely reworked scene in which the Tramp, and the audience, come to realize that she's blind, we see her respond to the slamming of a car door that we don't hear, at about 8:35 into the movie (there doesn't seem to be a good way to start just at that point in this post but you can click on this link to start it at just the right place: https://youtu.be/TkF1we_DeCQ?t=8m35s )
You'll see her react to the slamming door, but we don't hear it slam: we just see it.
I think it's important that we never hear what she's hearing. The Tramp can see but he can also hear and we can hear with him, just as we can hear with everyone else. We can see that she can hear but we never hear what she's hearing, which means that we focus on her bodily relation to the world, on her touch.
At the end of the movie, when she sees the Tramp for the first time, there's no reason for her to recognize him. But she would be able to recognize his voice, and that would be the natural recognition scene. And it wouldn't work.
What matters is that she should recognize him through touch, when unknowingly she gives him the change that she failed to give him before. Now she can see, and the days of touch are over: we can see them touch for the last time. All of this requires that the movie be silent, be a pantomime, so that the self-presence of sound doesn't enter into it.
It's another version of Orpheus and Eurydice, that central myth in which sight overwhelms sound -- the song that Orpheus sings -- with the result that touch is banished forever. He looks back at her and loses her forever, sees her going but will never touch her again. The happiest version is in Purgatorio: Dante looks back at Virgil, and he's gone, and so are his sweet songs; but he can truly see Beatrice now, though touch is no longer a relevant sense in the Comedy. City Lights isn't the saddest version, but it is an exceptionally clear exposition of that moment when the pantomime has ended.