Sound wont play in Unity

Hi everyone I'm trying to get a walking sound to play when you move with the game. I tried a lot of methods and the simple audio.clip=walkSound and audio.Play, and non of that worked. Any ideas on how to make the sound play?

public float walkSpeed = 5.0f;
public float slowPentaly = 0.5f;
public float gravity = 20.0f;
public float run = 2.0f;
private Vector3 moveDirection = Vector3.zero;
public AudioClip walkSound;

void Start()
{
            //transform.position = new Vector3(0,1.7f,0);
    }

void OnControllerColliderHit(ControllerColliderHit hit)
{
            if (hit.gameObject.tag == "Shard") {
                    moveDirection *= slowPentaly;
            Debug.Log("You are slow");
            }
    }
void Update()
{
            CharacterController controller = GetComponent<CharacterController> ();
            if (controller.isGrounded) {
                    moveDirection = new Vector3(Input.GetAxis ("Horizontal"), 0, Input.GetAxis ("Vertical"));
                    moveDirection = transform.TransformDirection (moveDirection);
                    moveDirection *= walkSpeed;

                        if (Input.GetKeyDown(KeyCode.LeftShift))
                            moveDirection *= run;
            }
            moveDirection.y -= gravity * Time.deltaTime;
            controller.Move (moveDirection * Time.deltaTime);

    }

}