April 27th excerpt:

Sam went to jump out as it came up next to his beam, almost tripping over himself in his surprise. It was just a mouse. He let out a laugh of relief as it sniffled at his jacket sleeve, bright eyes blinking up at him. “Holy crap, I thought you were a rat.” Sam brushed a hand over the rounded ears. Mice weren’t dangerous to him or Bowman. “You should be more careful. Where’s your family?”

( This came up once before *uses same answer* )

Actually, the size of ears plays less of a role in your ability to hear than the shape of those ears. Human ears are specifically shaped after millions of years of evolution to pick up the sound waves that are most important to us… mainly, the vibrations a human voice makes. Some people have better hearing than others, yes, but that is less dependent on their size.

So Sam might be able to hear a little better than Dean, just like it’ll be easier to deafen him by shouting, but it’s not a huge difference, and there are likely regular sized humans who might be able to hear just as good as Sam. There are a lot of people out there who can hear higher pitched tones while others can’t hear a thing.