They actually did this one on Mythbusters. As it turns out, a duck's quack has an odd waveform that tends to cancel itself out better than a lot of other sounds. What actually happens is that an echoing quack sounds just about like a non-echoing quack - that is, a duck's quack sounds like it's echoing even when it isn't, so if you're familiar with what ducks sound like, you won't notice it echoing.

Actually, don't read the above. It's terrible. Instead, read the synopsis at Wikipedia