In the film script:
Andy wrestles the phonograph player onto the guards' desk, sweeping things onto the floor in his haste. He plugs the machine in. A red light warms up. The platter starts spinning. He slides the Mozart album from its sleeve, lays it on the platter, and lowers the tone arm to his favorite cut. The needle HISSES in the groove...and the MUSIC begins, lilting and gorgeous. Andy sinks into Wiley's chair, overcome by its beauty. It is "Deutino: Che soave zeffiretto," a duet sung by Susanna and the Contessa. The prison comes to a halt.
The duet is from Mozart's "Marriage of Figaro." It is timeless, a universal thing of beauty. These inmates don't understand a word of it. But everyone stops working and talking in the yard. Everyone is frozen, hypnotized by Mozart. It reminds them of marriage, love, women, freedom--all that which is not Shawshank.
Andy is a martyr here for art. He is willing to punish himself to let freedom ring.
The scene is similar to the one in which he arranges the bottles of suds for the guys tarring the roof. Both of these scenes make Andy and the inmates feel human again. Shawshank has their bodies, but it does not have their souls.