Honestly, these are not wild animals, and many of them have been at the park for years, now, so they are pretty used to all of the humans in the park. With many of the animals' hearing capabilities, they can already hear guests scream from Kali River Rapids, and don't seem to care. Disney takes the best possible care of all of the animals, and as Evan said, they hear fireworks all the time--from the other parks (my dog can hear thunder about 10 minutes before I can...many of the animals have excellent hearing because most of them are either predators or prey in the wild, and need to hear the most minute sound to find a meal or escape being one). With the size of the enclosures these animals have, the lack of any predation whatsoever, and the great food they get (it's honestly better quality than you get at the Disney restaurants), they are content, which is apparent in the African elephants--it is very tough to get an elephant pregnant by artificial insemination in zoos, let alone by natural means; they are clearly happy.
