They aren't loud. Just louder than the females most of the time due to always quacking when I am around - something that my girls do not do. My drake is always quacking. It's just a whisper type of quack. You would have to watch some videos to understand if you don't have muscovies. My girls mostly just look at me and come to see what I am doing. They trill for treats (which is very quiet compared to a quack).
My drake is still much more quieter than a mallard derived breed.
My girls trill which is a very quiet type of duck "purr". And, they peep when worked up. I got a new rooster and they were all peeping when he was doing rooster business. The peeping is louder than the whisper quack, but you wouldn't hear it unless you are really close.
My girls will also honk if their tail is pulled and they don't like it.
I think the OP should pull up videos of as many ducks as she can.
I am thinking the quietest is still going to be the Muscovy though. As that was my conclusion when I researched the breeds that I was interested in.