[See, that would probably get more of a reaction out of him. That is, it would if it didn't sound so like everything he hears while he's possessed. Instead of a scream, it gets something like a strangled sob, during which Ghost almost gets control.

Now that Their voices have been pointed out, he can hear them; he legitimately had not noticed until now. ...Did we mention he's slow?]

..."Headcrab"? [Yep. That's his reaction, once he's finished internally flailing over that voice.]