The piece is 'Adagio For Strings' by Samuel Barber. It's an excellent example of the way in which music can be used universally to appeal to its audience, and arouse a specific emotion - in this case, a feeling of wistful sadness which is perfect for the situation it illustrates in 'Butterflies', Rea's longing for freedom and excitement which does constant battle with her feelings of responsibility as a wife and mother. This particular Adagio is very popular - you will find it on a number of mid-priced classical CD compilations at your local dealer.