I would assume it is a fever induced hallucination. It's not uncommon especially for children to have this with a fever.
My brother used to talk about chocolate houses every time he had a fever.
My son tells me to watch out for things that aren't there when he has a fever.
Treat the fever and if it gets above 103, be sure to get immediate medical attention.
Good Luck
I would say this is a hallucination ,how high is the fever, my opinion is he should be checked out by a Doctor.. .Is he hearing the voices before or after sleep ?