Did Mary Steenburgen actually sing for this?

They explicitly credit Mary with singing three songs in the films credits. I doubt they would have done this if they had then dubbed her, especially as at least one of those songs was part of the soundtrack and we don't see Mary singing it.

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Alex Cutler, Bath, England

Yes she sang. She has sung in other movies, but can't think of them offhand. It's funny that she sings so well given her speaking voice is odd.