You should most likely seek medical help. See a psychiatrist. It shouldn't be something you are ashamed of, millions of people regularly see psychiatrists. If you talk to him he may be able to help you, and if not, he may be able to determine if its something physiological and recommend you see a doctor and get medication. If none of that works and you still are hearing the voices, then I would suggest seeking out religion. Even though I'm skeptical about the whole possession deal, if none of the medical paths work, its never a bad idea to try something else.
Don't be afraid to reveal this information to someone who could really help. We can only give advice on what path to take on here, but none of us actually know whats best for you. Don't let fear or shame prevent you from getting professional help, because if you don't there's no knowing what could eventually happen. If the voices get more persistent it could drive you to hurt someone you love, or yourself. You need to get help before something like that happens.
Good luck