I love the moment in Shawshank Redemption when the Contessa/Susanna duet from Le Nozze di Figaro plays on the loudspeakers over the prison yard.

The overture to Verdi's La Forza del Destino played during the opening credit (and later sung by Elisabeth Depardieu during the film) of Jean de Florette sets the perfect tone for the film -- it's broody and gorgeous and telegraphs the tragedy from the get-go.