play sound when player reaches a certain speed

I want to have a sound play when I reach the speed of sound in my plane (721 mph). But my script isn't working.

I have:

var mph = rigidbody.velocity.magnitude *2.2369;
function FixedUpdate(){

if (mph = 721) {
audio.Play();
}

}

Any help?

You have to use == when making comparisons. = is only for assignments. Also, the odds of the mph variable ever being exactly 721 are practically nil. It should be

if (mph >= 721.0)