eNature - Bringing Nature to Life
Bird calls from a variety of species.
"The site's core content of wildlife information about almost 6,000 individual species is the same data set used to create the printed Audubon Field Guides. All the data has been carefully reviewed and vetted by leading biologists, zoologists and other natural history specialists."
(Going to the main page lets the user search for field guides to a range of animals. The site has advertising on it. A bookmarklet like "adblock" can make the site less distracting for the class if the content is being projected.)