How do I play audio once in Update?

Been trying to make an fps game and I have been trying to add a reload sound but this plays once every frame as it is in the update section. How do I make it play once?

using UnityEngine; using System.Collections;

public class Gun : MonoBehaviour {

 public float damage = 10f;
 public float range = 100f;
 public float fireRate = 15f;

 public int maxAmmo = 30;
 private int currentAmmo;
 public float reloadTime = 1f;
 private bool isReloading = false;


 public Camera fpsCam;
 public ParticleSystem muzzleFlash;
 public GameObject impactEffect;

 private float nextTimeToFire = 0f;

 public Animator animator;

 void Start ()
 {
     currentAmmo = maxAmmo;
 }

 
 void Update () {
     if (isReloading)
     //Play audio source here
     return;

     if(currentAmmo <= 0)
     {
         StartCoroutine(Reload());
         return;
     }

     if (Input.GetButton("Fire1") && Time.time >= nextTimeToFire)
     {
         nextTimeToFire = Time.time + 1f / fireRate;
         Shoot();
         AudioSource shoot = GetComponent<AudioSource>();
         shoot.Play();
     }

 }

 IEnumerator Reload ()
 {
     isReloading = true;
     Debug.Log("Reloading...");

     animator.SetBool("Reloading", true);

     yield return new WaitForSeconds(reloadTime);

     animator.SetBool("Reloading", false);

     currentAmmo = maxAmmo;

     isReloading = false;
 }

 void Shoot()
 {
     muzzleFlash.Play();

     currentAmmo--;

     RaycastHit hit;
     if (Physics.Raycast(fpsCam.transform.position, fpsCam.transform.forward, out hit))
     {
         Debug.Log(hit.transform.name);

         Target target = hit.transform.GetComponent<Target> ();
         if (target != null) 
         {
             target.TakeDamage (damage);
         }

         GameObject impactGo = Instantiate(impactEffect, hit.point, Quaternion.LookRotation(hit.normal));
         Destroy(impactGo, 2f);
     }
 }
     

}