как воспроизвести звук во время клика в Unity?

не работает audio.PlayOneShot

Ошибка:

NullReferenceException: Object reference not set to an instance of an object PlayerMove.Update () (at Assets/Script/PlayerMove.cs:29)

Скрипт управления персонажем:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Audio;
public class PlayerMove : MonoBehaviour {
    public float speed;
    private Vector3 dir;
    public AudioClip impact;
    AudioSource audio;
    // Use this for initialization
    void Start () {
        dir = Vector3.zero;
    }
    // Update is called once per frame
    void Update () {
        if (Input.GetMouseButtonDown(0))
        {
            speed=20;
            if (dir==Vector3.forward)
            {
                dir=Vector3.left;                   
                audio.PlayOneShot(impact, 0.7F);
            }
            else
            {
                dir=Vector3.forward;
            }
        }
        float amoutToMove = speed*Time.deltaTime;
        transform.Translate(dir*amoutToMove);
    }
}