My theory (do you agree?)

While Maggie Smith may be a fine actress, she probably sings about as well as the average actress, certainly not like the character she plays? Notice the one time they showed her singing, it was silent and you saw the piano. The next time she sang (right at the end of the movie) they just showed the house. I bet that was deliberate because as there may be nothing horrible with her singing, it is not diva quality? Do you agree?