How do I discuss the voice of this short story?
In any work of literature, looking at the "voice" of the work involves considering the way in which the story is presented. You could think of "style" instead of "voice" if that helps you approach the question.
Start by determining the point of view from which the author has presented the work. In the case of A Good Man is Hard to Find, the story is presented from a third person perspective - someone not directly involved in the action is telling the story. However, insight into Grandmother's thoughts is somehow available and included in the telling of the tale.
Is the story told in very formal language or in a more informal pattern of speech? Does the author use lots of descriptive language? If so, what are examples? Is there detailed explanation of the action or does the author leave lots to the reader's imagination? Is presentation of the plot fairly sequential through the time involved in the action, or does it jump around in time or location? All these considerations are part of the author's voice in telling this story.