Formatting your code using the 101/010 button will make it far more likely that folks will read and answer your question. I formatted it for you this time. There are a number of reasons that this code might be failing. Here is a checklist of things to verify or try:
Make sure there is an AudioSource attached to this game object.
Make sure the audio source has a clip assigned and the clip will play
Make sure that one or the other of the two object colliding have a Rigidbody
Put a debug statement inside the OnCollisionEnter() to verify that it is being called.
Output the relativeVelocity.magnitude for all collisions to verify that you are getting collisions of magnitude greater than 2.0.