Re: A Duck's Quack Doesn't Echo? Surely a hoax?

This sounds like an "urban legend" or "urban myth". These are ideas that spread 
through a culture with little objective basis. They are not necessarily false, 
although this one clearly is. There is a newsgroup devoted to investigating and 
debunking these. Point your newsreader at alt.urban.folklore

See also the 

You could get all the information you could about this myth: origins, places 
first posted, etc and submit it to the newsgroup. Be sure to read the FAQ from 
"cover to cover" before posting. Fame and virtual fortune await you.

Regards,
David Winsemius