"Voi Che Sapete" is the Italian opera aria playing on the soundsystem of the Elizabeth when Sayid, Jack, and Sawyer find Desmond inside, in "Live Together, Die Alone, Part 1". It is an aria from Mozart’s Le Nozze Di Figaro (The Marriage of Figaro), written by Lorenzo da Ponte. The character who sings it is Cherubino, a lovesick young pageboy. The topic of love seems appropriate for Desmond, as the rest of the episode delves into his relationship with Penelope Widmore.
Voi che sapete che cosa e amor,
You who know what love is,
- Wikipedia - "Voi Che Sapete"