There's only one explanation I can think of, which involves some spoilers:
In the final scene we discover that George has a thick French accent. This helps explain his reticence to move into talky films, which up until this point we thought was only down to pride. Perhaps all along George believed that audiences would want their talky stars to have all-American accents, hence his clinging on to the silent era. (This theory is also supported by the fact that neither Peppy nor the studio boss argues with George's assertion that nobody would want to hear him talk.)
So I wonder if what's happening in this scene is that George doesn't really hear what the cop is saying (hence the lack of title card), all he hears is his American accent: the one thing George can never have, and which he believes has cost him everything. After this scene he goes back to his burnt-out apartment, sits at his bureau, and faces start appearing round him: again, they're talking but we get no title cards, and again it ends up zooming on just on the mouths.
What the people are saying isn't important: it's the fact that they're talking (or how they're talking) that's significant. George has become completely preoccupied with his accent, tormented by how effortlessly other people do what he can never do (speak with an American accent), and it is this that drives him to contemplate suicide.
The cop is just the catalyst for this final downward spiral.