MonoBehaviour.OnCollisionEnter(Collision)

Description

OnCollisionEnter is called when this collider/rigidbody has begun touching another rigidbody/collider.

In contrast to OnTriggerEnter, OnCollisionEnter is passed the Collision class and not a Collider. The Collision class contains information about contact points, impact velocity etc. If you don't use collisionInfo in the function, leave out the collisionInfo parameter as this avoids unneccessary calculations. Notes: Collision events are only sent if one of the colliders also has a non-kinematic rigidbody attached. Collision events will be sent to disabled MonoBehaviours, to allow enabling Behaviours in response to collisions.

#pragma strict
var audio: AudioSource;
function Start() {
	audio = GetComponent.<AudioSource>();
}
function OnCollisionEnter(collision: Collision) {
	for (var contact: ContactPoint in collision.contacts) {
		Debug.DrawRay(contact.point, contact.normal, Color.white);
	}
	if (collision.relativeVelocity.magnitude > 2)
		audio.Play();
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
	AudioSource audio;
	
	void Start() {
		audio = GetComponent<AudioSource>();
	}
	
    void OnCollisionEnter(Collision collision) {
        foreach (ContactPoint contact in collision.contacts) {
            Debug.DrawRay(contact.point, contact.normal, Color.white);
        }
        
        if (collision.relativeVelocity.magnitude > 2)
            audio.Play(); 
    }
}

OnCollisionEnter can be a co-routine, simply use the yield statement in the function.