I noticed last night DS doesn't startle at loud noises, or turn to see where a noise is coming from. I was trying to get his attention from a few feet away, and noticed he wasn't trying to see where the noise was coming from. I tried banging a pot lid with a spoon (he was in his exersaucer, I was several feet behind him), snaping my fingers close to his hear, DH slammed a book on our coffee table (a few feet away from him), and nothing. He didn't turn to see what caused the noise, didn't turn his head, he didn't jump or startle.
I don't know anything about baby hearing development, but does your LO do something similar? He does laugh, coo, and make sweet little noises, so I'm guessing something is working in his ears..