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);
    }
}

}

Just add a condition for it:

private bool isReloading = false;
private bool playReloadSound = false;

void Update () 
{
    // ...
    if (isReloading && playReloadSound)
    {
        //Play audio source here
        playReloadSound = false;
    }
    // ...
}

IEnumerator Reload ()
{
    // ...
    isReloading = true;
    playReloadSound = true;
    // ...
}