Well, sort of. The reason it's such a great horror story is because we never know for sure. The voice was supposed to be beelzebub, the lord of the flies, basically Satan. The whole story seems to take place in the real world, so having something supernatural happen that late in the novel adds a sort of terror to it. It means that there's possibly something else happening on that island, below the surface. But we never know for sure, because the author hints that Simon may just be crazy. The thing is, Simon is the only thing standing in the way of Beelzebub's plans, and he's the only one who realizes what's happening. When he tries to warn the other children, he's killed. That's the moment when things finally got out of control, and there's no turning back. Golding raises a lot of questions, but he never offers any answers. It's kind of like Jacobs ladder, or other horror stories, where you're questioning the characters sanity. Are we alone in the universe, or are there dark forces working behind the scenes? In Lord of the Flies both possibilities are terrifying. Either way, literally or figuratively, Beelzebub won.
It reminds me of Berserk, where the author hints at supernatural things existing, but we never see them until the end. Or Game of Thrones, where supernatural things exist in the shadows, but aren't seen by normal people.