Old thread buy I want to add that some regular (non-Muscovy) breeds can definitely be quiet, but it can depend on your source. If you must have very quiet ducks, you should buy from a breeder or farm that has multiple breeds, and ASK them which of their breeds is the quietest!
We are in a city environment where the zoning bans "quacking ducks." So, we asked Metzer (in California) and they said their Cayugas are quietest, then Welsh Harlequins. We got 1 Cayuga and 1 Buff and the Cayuga is indeed quiet; the Buff is pretty quiet.
Our ducks would occasionally quack loudly at random times back when they were adolescents, before they started laying. Now, the Cayuga is always quiet and the Buff is loud only when we're trying to move her quickly somewhere, or when she's anticipating me tossing her some greens in the morning.
Our ducks are quieter than most chickens. Many chickens get loud when they're laying an egg, and also sometimes randomly during the day just start cawing. Our ducks never randomly quack anymore, and they always lay early (and quietly) in the morning before we let them out. (that means blue jays or crows never have an opportunity to "poach" an egg) Super prolific so far--two eggs every morning for 6 months now!