I'm curious if there is any good reference for animal calls (wolf's howl, horse's whinney, etc.), specifically North American animals, and how far their calls can be heard. Thanks!

I can give you a few sites:
http://members.tripod.com/Thryomanes/AnimalSounds.html
Animal Calls (http://www.ultimatefieldguide.com/animal_calls.htm)
ANIMAL INFO - Animal Sounds Library (http://seaworld.org/animal-info/sound-library/index.htm)
Macaulay Library - Animal Sound & Video Catalog (http://www.animalbehaviorarchive.org/loginPublic.do)
Elephants Call Long-Distance After-Hours (http://news.nationalgeographic.com/news/2004/03/0303_040303_elephants.html)