Here's the thing; a lay person would hear that and say, "Uh oh, that's psychosis." Hearing voices, especially aggressive voices that come from "outside" his head are definitely symptoms of schizophrenia. I think 11 is a little young for onset schizophrenia, but, as everyone here has said, you need a professional opinion.

However...

My brother went through something very similar when he was about 10 or 11. He had gone to spend the summer with our Aunt and Uncle, who are very religious. They had filled his head with all sorts of nonsense. Don't get me wrong, I am a Christian and I am spiritual, but they crammed his head full of so much frightening BS that he had night terrors for months after he came home. He said he thought about death all of the time and he could hear Satan whispering to him. After about a year, it all passed and he is probably the sanest of my siblings.

I know you said you don't put that kind of thing in front of him, but is it possible that there is some kind of outside influence? I don't think you should blow off having him evaluated. At the same time, I don't think you need to worry yourself sick over it. Remember, kids can almost always tell when you are upset about things.

Hope this helps.