My neighboor complained about our rooster crowing at 4 am so we took action. Every evening when they go to roost, I go in the coop, grab the rooster, and put him in a small carry kennel designed for a small dog or cat. Then, I bring him up by the house and cover him with an old sleeping bag under a shed. He barely crows in there and it is greately muffled. So, happy neighboor.
But the rooster is getting ticked off. But I want to keep him because, contrary to conventional wisdom, I have noticed I do get more and bigger eggs with a rooster servicing my hens. And this boy is very romantic!
But I am definetly looking into building a new coop with sound proofing for him to stay in. The ventilation issue can be done by creating a baffle of sound proofing that the air can flow around, but the sound waves will strike and reflect back towards other soundproofing. Basically, ya want the sound waves to reflect and scatter off of as many surfaces as possible in the room while being absorbed by sound proffing each reflection.