The problem is... (SPOILER)

The scene, where Michael Caine sits in his room talking to the Queens emissary about conducting his piece, with Weisz in the background. And he continues:

"... that I composed simple songs for my wife. And only she - has ever performed them, and only she has recorded them. And as long as i live, she will be the only one to sing them. The problem is dear sir... My wife can't sing anymore"

And then we see Weisz, sitting behind him, covering her mouth to hold in her crying. Damn that was a moving scene! So beautifully written and acted. Especially with the scene in the spa from moments before, where Weisz gives Caine a yelling about not caring for his wife, in mind.