Basically, if a gunshot were fired inside a large cave (hypothetical), then you wouldn't just want the initial sound. How could you make it produce an echo? I am thinking you may be able to bind the audio with a wave emitter traveling at a certain speed in all directions, then use a raycast to detect collisions with boundary's proportional to the energy left in the residual sound waves.
For Unity 2.6 you could always try FMOD. There are a few .NET DLL plugin's (pro Unity license required) that you can use. This will allow things like doppler / flange / delay etc....
Here's one: http://www.squaretangle.com/FMODUnity.html