Acoustic Ecology is the study of the relationship between living things and the environment through sound. In a an area such as Otari Reserve, with lots of trees and plants, we can often hear birds and insects but don't see them. Making field recordings can allow us to identify bird and insects from their calls, even if we cannot see them. As a means to study an areas biodiversity, sound is another useful tool and can provide another way to experience an environment.
The Macaulay Library has huge collection of animal sounds and videos from around the world and be warned you can spend hours listening to various animal sounds and forget about the work you are supposed to be doing.
Here in New Zealand a few years ago there was a competition run by DoC to remix bird calls and you can listen to the results here.
There are many other websites and resources on the web to search for sound recordings, as well as many artists who have used sound in their work and I will post some examples to get a flavor of the different ways they have used sound.