The music is from "Le nozze di Figaro (The Marriage of Figaro)" by Wolfgang Amadeus Mozart, the piece is called ."Duettino - Sull'aria".
You are right in saying it's from the Shawshank redemption.
Have a look at "You Tube" and search for "Duettino - Sull'aria", you will find Dame Kiri Te Kanawa & Ileana Cotrubas singing it.