I think plain and simple it's a "fourth-wall" thing. He was waiting for the dialogue card to roll in when the policeman started talking to him, as was the premise of the movie. When this didn't happen he was confused, it felt as if he was ostracised not only from the hollywoodland world, but from this same "The Artist" picture altogether. Thus he decided it was time to call it quits. It's a meta-thing. To me, the "accent" explanation is contrived and the repeat of the "disorienting live sound" explanation doesn't make sense as a vehicle at that point, it's regressive. So the key to interpret the scene is the absence of dialogue-cue cards. Notice how he takes his eyes away from the policeman and where the director or crew of the film is supposed to be, almost desperately waiting and ponderding as to why they are taunting him with not doing their job and not providing cards. He's like "oh, now you wanna make THIS movie a talking one, screw you then, I'm out of here". The resolution of the film then comes, not only in terms of the story, but very much so from this movie remaining a silent one after the averted suicide (notice the BANG card, there weren't any "sound-descriptive cards" up to then, also the making-up scene is eeriely silent despite the windy conditions, barking, shouting, crying, laughing, etc.). Then comes the accent thing, as a twister on that. But only as a twister. If the accent thing was all that important, the big producers would never ask of George to transition in the first place. To suggest it as a main premise of the film and not a twister, to me, it's "thinking-too-hard" and also a bit racist as well. Accent wouldn't have been a thing if he wanted to transition. In the end, the artist compromises with another form of pure "art", the dance. And then it's "let there be sound".
An excellent movie with a transcending view of the all important concept of "silence" in the mind of the artist and the intellectual as a stark contrast and duality with "expression".