Well, technically no one "knows" for sure who it is. MANY people think it is Samantha, but that woul mean she would be contradicting herself. When she says, "So you hear, the voices?" she is talking about that. Saying that means that she is NOT the person (or entity) saying "Max Ammo!" and so forth. I believe it is some other character (possibly even Maxis) that we haven't even heard of saying those things. Someone that has EXTREME importance in the storyline.
I love how this turned out, but you guys left out some important details. First, the picture frame with the blacked out picture was supposed to be a picture of a mexican edward richtofen killed when he was recruiting some people such as nickolai and taeko. Also, agartha is M.P.D(dont know what it stands for) which a device used to control the zombies. Thats where they teleported sam to. Dr. Maxis's wife was most likely Sophia, because she was attracted to him.