This thread cracks me up! Love illicit stealth chickens. Picturing them in those glasses with a fake mustache attached to fool the bossy neighbour who doesn't like bending bylaws. Good luck!
The noisiest of my pullets became one of the quietest after she started to lay and one of the quietest took over as the noisiest. You can't really tell who will be quiet until they settle in to laying.
How about a privacy fence? leave a few feet between the coop and the fence, landscape with a few grape vines or juniper bushes and if you have a noisy girl you can add hay bales (not straw) for noise . However, when my buff orpington sees a dog she goes on forever, you could rehome anyone too loud.
Are you allowed to have parrots as pets? Download some of the louder ones, play that when your girls are singing. No one will hear the chooks. A friend of mine did this, other people in her neighborhood have loud parrots. So no one thinks anything of the screaming, and no one knows she has her hens.