AudioSource not playing audio

For some reason the audio will not play even though it is being called upon in the code. Here is the code:

using UnityEngine;

using System.Collections;

//Update, the script is now attached to a zombie model animation code is added

public class enemy : MonoBehaviour {

 private GameObject wayPoint;

 //public float Cube;

 private Vector3 wayPointPos;

 public Transform zombie_dist;

 //This will be the zombie's speed. Adjust as necessary.

 public float timer;

 private float speed = 3.0f;

 public Rigidbody rb;

 public GameObject zombie;
 Animation Death;
 public float dist;
 //public CapsuleCollider cc;
 public Color newColor;
 public bool isClose;
 public Transform target;
 public float hitBox;
 public bool isDead;
 public bool isIdle;
 public float deathTime; // how long the zombie lives after its first initial death;
 
 //public float attack;
 
 
 public  AudioSource attack_audio;
 void Start ()
 {
     attack_audio = GetComponent<AudioSource> ();
     //Attack =  attack_audio.GetComponent<AudioSource>();
     isIdle = false;
     deathTime = 5;
     isDead = false;
     isClose = false;
     Death = zombie.GetComponent<Animation> ();
     timer = 10;
     //cc = GetComponent<CapsuleCollider>();
     rb = GetComponent<Rigidbody>();
     //At the start of the game, the zombies will find the gameobject called wayPoint.
     wayPoint = GameObject.Find("wayPoint");

     //move = GetComponent.<Animation>();

 }

 
 void Update ()
 {
     if(target != null)
     {
         transform.LookAt(target);
     }
     if (zombie_dist) {
              dist = Vector3.Distance(zombie_dist.position, transform.position);
             print("Distance to other: " + dist);

     }
 if (dist <= 1.5 && isClose == false) {
         speed = 0;
         Death.Play("Attacking");
         attack_audio.Play();

     }

     if (dist >= 8 && isClose == false) {
         speed = 0; 
         Death.Play("Idle");
     }
     if (dist >= 1.6 && isClose == false && dist <= 7) {
         speed = 3;
         Death.Play("Walking");
     }


     if (isClose == true) {
         timer -= 1 * Time.deltaTime;
         speed = 0;
     }
     if (timer <= 0) {
         Destroy(gameObject);
     }


     wayPointPos = new Vector3(wayPoint.transform.position.x, transform.position.y, wayPoint.transform.position.z);
     //Here, the zombie's will follow the waypoint.
     transform.position = Vector3.MoveTowards(transform.position, wayPointPos, speed * Time.deltaTime);
 }


 //void OnTriggerEnter (Collider other)
 //{
 //    if (other.gameObject.tag == "bullet") {
 //        gameObject.GetComponent<Renderer>().material.color = Color.blue;
 //        rb.isKinematic = false;
 //        speed = 0;

// cc.isTrigger = false; // newColor = Color.black; // } // } void OnCollisionEnter (Collision col)//if there is a collision with the prefab projectile the the color of the enemy will be blue (M4A1 gun model compadible) { if (col.gameObject.name == "projectile(Clone)") { gameObject.GetComponent().material.color = Color.blue; //rb.isKinematic = false; speed = 0; newColor = Color.black; Death.Play ("Dying"); isClose = true; gameObject.GetComponent().enabled = false;

     }
     else if (col.gameObject.name == "bullet_2(Clone)") {
         gameObject.GetComponent<Renderer>().material.color = Color.red;
         //rb.isKinematic = false;
         speed = 0;
         newColor = Color.black;
         Death.Play ("Dying");
         isClose = true;
         gameObject.GetComponent<BoxCollider>().enabled = false;
     
         //if the ak-47 model is enabled the the gun will shoot the prefab bullet_2 which make the enemy color red
     }

 }

}