Mute swans are called that because they are less vocal than other species of swans. They do make a variety of printing, course whistling and snorting noses and has said predators that try to do their territory. Some swans will his sat and try to even attack humans. To me the sound that is best associated with the mute swan is the vibrant throbbing of its wings while it is flying. If they fly over you, and they are not far away, you can feel the force and sound of their wing beats. It is very impressive, it feels like it is vibrating through your whole body.
An interesting fact is that a group of swans are generally known as a flock, but in the Oxford English Dictionary, when swans of flying in a "V" formation. They are known as a Wedge of Swans