Watch on the official Unity learn pages here -
http://unity3d.com/learn/tutorials/modules/beginner/5-pre-order-beta/duck-volume-audio-effect

In this lesson we'll look at Unity's Duck Volume audio effect. Duck Volume allows us to cause the volume of an audio signal to duck or get quieter based on the volume of that signal, or a different one. A common use of this effect is sidechain compression. We'll look at how to set up Duck Volume including routing signals to control ducking with Send audio effects.