[nomadbrah]

Did you ever tell him, that you think he is acting weird? His reaction to that would tell you a lot. If he sort of smirks and acts as if he is in some secret and you're not, then that is another clue for hearing voices. Maybe there really was a simple explanation. In any case, trust your instincts and don't be the one to make the decision. Good to bring it up with the other flatmates as well.