These days you have a lot of people saying that they get messages from a spiritual element, saying that they know exactly what goes on in heaven and hell. I don't believe a word of it. I think that the narrator was one of those guys. He had some idea, telling him to kill the old man, and from that point, he couldn't stop thinking about it, until he had done the deed .
Personally I think that the narrator was losing his mind, slowly, but surely. He thought that he could get away with the crime, but couldn't, because of that sense of hearing. The same sense of hearing that he thought was a blessing, but turned into a curse, putting him in jail. It just goes to show, Karma always comes back.