Sound volume based on rigidbody velocity before collision.

off the top of my head something like this...it doesnt work but it might help put you in the right direction (hopefully)

EDIT: Fixed ambiguous reference, still will crash unity though

 //unstable, will crash Unity
 
 @script RequireComponent (AudioSource)
 
 var collide : AudioClip[]; //array of audio clips
 
 var audioVolume : float; //volume
 
 var audioLength : float = 1; //length
 
 function OnCollisionEnter(collision : Collision) { //on collision
 
 audio.clip = collide[Random.Range(0, audioLength )]; //random audio clip from array
         
 audio.volume = collision.relativeVelocity.magnitude; //set volume to speed of object
 
 audio.Play(); //audio play
 
 }
 

This is very old. I'm trying to solve this problem too, but anyway the code above has an initial problem: volume must be a value between 0 and 1, where 0 is 0% of the volume of the clip and 1 is 100%. So if you do audio.volume = collision.relativeVelocity.magnitude; this will fail because the velocity magnitude can be greater than 0. You can use Mathf.Clamp01 but if most of the times velocity on collisions is higher than 1, all those collisions will have volume = 1 so you will notice no difference in a collision with velocity = 2 and velocity = 20. If velocity on impact is random (the object can vary a lot its own velocity) you must get a max value (aprox) and divide by it. For example if most of hits are in the range of 5 - 20 velocity, you can do:

 audio.volume = Mathf.Clamp01(collision.relativeVelocity.magnitude / 20);