How is singing diction different than speaking diction?
They start out basically the same, but most of the guidelines for sung diction are based on the guidelines for what’s known as poetic declamation. Especially in French, the way poetry is read differs from ordinary speech. For instance, there are more required liaisons in poetic declamation (and thus in singing) than there are in speech. Second, some changes occur precisely because we are singing the language. We may for instance modify some pronunciations because the pitches they are sung to are either high or difficult. And some spoken sounds, like the Parisian “r,” are changed to make them more pleasing to the ear.