Trouble changing sound when going through a collider.

I'm not great with coding so go easy on me. I have a script on my character that controls the movement and also plays the footstep sound. I want to change this sound when the player goes through 'portals' (so basically when the player hits a collider). The movement script is called 'physicWalk' and the variable is called 'footstepSound'.

I've tried making a really simple script that will change the variable on collision but i'm getting the error: An object reference is required to access a non static member. This is what I have so far

  public class FootstepSelector : MonoBehaviour

  {

 public AudioClip FSound;

 void OnCollisionEnter()
 {

     physicWalk.footstepSound.LoadAudioData (FSound);

 }

}

As you can see I want to be able to load different sounds as there are many different colliders leading to different surfaces.

Thanks in advance