From Sandra in Sydney: According to report some folks use rude words in pre-and-post-operative mode. Singing sounds much better.
I think it's because, while under the influence of the anasthetic, the conscious mind stops putting effort into maintaining the mask of niceness we're expected to wear in public, and folks let loose with the anger and frustration they feel.
If Mary was singing, it means she's just as nice and kind and basically joyful behind the mask as in front of it...