// Play a sound when we hit an object with a big velocity
function OnCollisionEnter(collision : Collision) {
    if (collision.relativeVelocity.magnitude > 2)
        audio.Play();
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnCollisionEnter(Collision collision) {
        if (collision.relativeVelocity.magnitude > 2)
            audio.Play();
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnCollisionEnter(collision as Collision):
        if collision.relativeVelocity.magnitude > 2:
            audio.Play()