I feel like a geezer saying I can't remember exactly, because I just saw the film yesterday, but I think I know what you're talking about. I also noticed a difference in the tone, but I think it could just be that Emmy's singing voice is actually stronger than it seems on film (likely the director's choice to make her more breathy and weak - I mean the girl's sung at the Met, but her voice in the film would likely not even get her in the door!) and the title song is her singing as she really is.
I'll listen to the soundtrack again and see. Sorry if that sounds incoherent. I'm breathing in bleach right now. But if Emmy's voice sounds stronger on "POTO" than the rest of the movie, I'd bet that's her singing "like normal". (Esp since people yesterday said her voice sounded much better on GMA than it does in the movie.)
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