You could probably fix this by examining the relativeVelocity reported by the collision in the OnCollisionStay method, and playing a sound if it exceeds some threshold. You could even change the volume of the sound based on the relativeVelocity.

Something like this:
(for illustrative purposes! - untested, may contain errors!)

var FloorSound : AudioClip;
var LockerSound : AudioClip;
var impactSoundThreshold = 0.5;  // tweak this value

// both the "Enter" and the "Stay" collisions pass
// their data to the sound function:

function OnCollisionStay (collision : Collision) {
    PlayCollisionSound(collision);
}

function OnCollisionEnter (collision : Collision) {
    PlayCollisionSound(collision);
}

function PlayCollisionSound (collision : Collision) {

    // check if the collision was hard enough to generate a sound
    if (collision.relativeVelocity > impactSoundThreshold) {

        // select the correct sound effect based on the object's tag
        switch (collision.gameObject.tag) {
            case "Concrete": audio.clip = FloorSound; break;
            case "Locker": audio.clip = LockerSound; break;
        }

        // calculate the volume (anything above 4*Threshold = full volume)
        var volume = Mathf.InverseLerp(impactSoundThreshold, impactSoundThreshold*4, collision.relativeVelocity);

        // play the audio
        audio.volume = volume;
        audio.Play();
    }
}

Figured it out now. It's very similar to what Duck sugested.

var LockerSound : AudioClip;

function OnCollisionStay (collision : Collision) {
        if (collision.relativeVelocity.magnitude > 0.5) {
            audio.clip = LockerSound;
            audio.Play();
            audio.volume = (collision.relativeVelocity.magnitude / 5 + 0.5);
            audio.pitch = (Random.value * 0.5 + 0.5);
            }
        }

I decided to take out the difference in sound files as this was complicating things a little and made some funny sounds but you can add it back in quite easily.

I am not a coder but I had something similiar ... If your locker is always on the concrete then you shouldn't use the collision with the concrete to activate the sound. Try to link it when you character is pulling it iPushLocker = true;. you should also consider looping the sound while you push...

var iPushLocker : boolean = false;


function OnCollisionEnter (other : Collision) {



 if((other.gameObject.tag == "Locker") && (iPushLocker == true ))
    {
        audio.clip = FloorSound;
        audio.Play();


    }


}

and then add onCollisionExit to set the IPushLocker as false. Hope it helps a bit.