The Audio Listener, simply put, is a component designed to work exactly like a pair of human ears. Its purpose is to hear the sounds in the application world and transmit that to the user, through their speakers.  

Much as your head can only be in one place at a time, in Unity, there can only be one active Audio Listener at a time. There may be times when doing sound tricks may require multiple listeners turning on and off. In those cases, if you have more than one Audio Listener active at the same time, you will get an error.  

The Audio Listener is a component that is automatically created and attached to the Camera object on its creation. Generally speaking, you will never move this from the Camera object. Also, due to its simplicity of purpose, the Audio Listener component does not have any options to change.