Sop HDR Audio package is an implementation of the high dynamic range (HDR) audio manager for Unity 3D.
It is free and comes with the full source code.
Sop HDR Audio DEMO
The HDR audio is a dynamic mixing technique that allows relatively louder sounds to drown out softer sounds.
It can greatly simplify work on a scene with many dynamic sound sources as
the volume and priority of each sound in a scene are set automatically by the HDR audio manager - based on loudness values assigned to each of these sounds by the developer.