Muscovies do not quack, they make trilling, cooing, hissing and huffing noises. the only time mine have ever quacked was when somethign scared them or one of the others pinched them.
males will be obviously bigger than females, especially the feet. Males tails are longer and wider while females have shorter, pointy tails.
Hope i helped
They definately make that trilling noise!! both of mine are about the same size...do you have a pic of a male and female that I could compare to?