A duck's quack does not echo, and nobody knows why. It is one of the all-time great trivia nuggets, passed around as a genuine unexplained mystery of physics, the quack that defies sound itself.
A duck's quack absolutely echoes; it just does not sound like an obvious echo, which is the whole reason the myth survives. Sound is sound, and there is no physical law that would let a duck exempt itself from reflection. An acoustic engineer at the University of Salford, Trevor Cox, actually put a duck named Daisy in a special echo chamber and a reverberation chamber and recorded her quacks. The echo was there, measurable on the equipment. The catch is that a quack is a fairly quiet, gradually fading sound with no sharp start or stop, so the echo blends into the tail of the original and your ear struggles to separate the two, unlike a hand-clap or a shout that has a crisp edge. So the mystery has a dull but solid answer: it echoes fine, the echo is just camouflaged by the soft, drawn-out shape of the quack. No physics broken, no ducks exempt.