I've had a theory for a while about why Samuel can hear Richtofen. I think it's because he's eaten zombie flesh before. After listening to all of his quotes, I think it's quite clear.
He calls zombies meatlockers, mindless flesh burritos, and stuff like that.
I also remember Abigail saying something like this, "You ready for some flesh Stuhlinger, cause I'm about to drop a lot of meat".
Here are some quotes:
"You don't bite me, I bite YOU!"
"That's right. She's next. Always being too nosy. Asking about who I'm talking to, who I'm eating. No, no! Shut up!"
"Okay, now you're just making me hungry!"
There are also a couple of quotes from the TV in the farm house:
"I'll do it. I'll do it for you. For you and the FLESH!"
Presumably he's listening to Richtofen's instructions.
"The other voice(Richtofen)cannot be heard by humans who have not (static noise makes the voice unclear)".
I think he says they cannot be heard by people have not eaten zombie flesh.
If this theory has been posted before I apologize. Tell me what you guys think!