[HeyLaughingBoy]

It's a bit off topic, but having raised ducks, I think I know the reason for that "fact."

Duck quacks have an odd sound sometimes, almost like an echo in itself. I've only noticed it when they're on the ground; in the air they seem to quack differently. Anyway, with a quack that already sounds a bit like an echo, I could see a case where the quack was actually echoing against a hard surface, but not sounding any different from the "normal" quack.

My ducks flew away years ago, so I can't do any scientific testing now...