I would talk to your doctor at your next check-up, but it may be nothing. My son was the same way. He could sleep through a train wreck! Doorbell ringing, dog barking and he wouldn't even flinch. I wouldn't shrug it off completely, but I wouldn't panic just yet, he is very young and I doubt there's much intervention they can do already even if he did have some hearing impairment. Try and observe him throughout the day, does he ever react to sound? It sounds like you have a very busy house with all those kids and he may have gotten used to it, even from the womb and it's not as noticeable to him.