The beautiful Mozart duet from the famous Mozart opera that Andy plays for the prisoners represents everything that the prison lacks: beauty, a world outside the confines of the prison, sharing the wealth. The men listening seem to sense that and we see them looking up in wonder at the sounds coming over the loudspeakers—speakers that are probably used only for broadcasting threats and institutional demands. It's like they don't even know what to do with it, but eventually give themselves over to the music.
You have to wonder if it's fair for Andy to give them a taste of something they'll probably never experience again. Is it better not to know in the first place?
Fun fact: the duet is about a woman planning to expose her husband's infidelity.