The voice is a symbol of hope amidst the destruction of war. Mr. Morgan, the defrocked priest formerly of the parish, has been hiding in the church, seeking sanctuary from the increasingly deadly German air-raids. Unfortunately for Morgan, this isn't enough to protect him, and the church suffers a direct hit from a German bomb. Morgan is buried beneath the debris, and while his successor—Reverend Lewis—and others try to rescue him, he keeps up his spirits by singing.
Lewis is positively mesmerized by the sound of Morgan's voice, so much so that he's prepared to forgive him for his transgressions. While trying to rescue Morgan, he falls into a tunnel, but fortunately isn't badly hurt. It's then that Morgan explains to Lewis that he sings because he is afraid. Lewis realizes just what he means and soon joins him in song, forming a veritable choir beneath the mounds of rubble.
As well as symbolizing hope, then, the voice also represents fear. Or, to be more precise, it represents the fine line between hope and fear. In the case of Morgan and Lewis, the singing voice expresses the hope that they'll be rescued, as well as the fear that they won't.