Thanks choices. You're right they can be noisy - I once had to climb off the river and carry my canoe round a swan's nest due to its anger at my presence appearing to threaten the little ones - it was far from mute then! I looked up why 'mute' - this is what the BBC site says:
"Mute swans are so-called because, compared to other kinds of swan, they make very little noise. However, they have a few distinctive calls, such as a hissing noise when scaring away an aggressor, a snorting 'heeor' call and a gull-like 'ga-oh' sound".
So now I know!