How can i find loudness of microphone from code? (code not working)

I followed a tutorial on how to move an object by your voice and there is this line of code which is important that doesn't work. Whats the problem? video

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class gun : MonoBehaviour
 {
 
     public float sensitivity = 100;
     public float loudness = 0;
     AudioSource _audio;
 
     public float damage = 10f;
     public float range = 100f;
     public float force = 700f;
 
 
     public GameObject hitEffect;
     public ParticleSystem muzzle;
     public Camera fpsCam;
 
 
 
     private void Start()
     {
         _audio.GetComponent<AudioSource>();
         _audio.clip = Microphone.Start(null, true, 10, 44100);
         _audio.loop = true;
         _audio.mute = true;
         while (!(Microphone.GetPosition(null) > 0)) { }
 
         _audio.Play();
 
     }
 
     // Update is called once per frame
     void Update()
     {
         loudness = Volume() * sensitivity;
 
         if (loudness > 9)
         {
             Shoot();
         }
 
     }
 
     void Shoot()
     {
 
         muzzle.Play();
 
         RaycastHit hit;
 
         if (Physics.Raycast(fpsCam.transform.position, fpsCam.transform.forward, out hit, range))
         {
             Debug.Log(hit.transform.name);
         }
         if (hit.rigidbody != null)
         {
             hit.rigidbody.AddForce(-hit.normal * force);
         }
 
         Instantiate(hitEffect, hit.point, Quaternion.LookRotation(hit.normal));
 
     }
 
     float Volume()
     {
         float[] data = new float[256];
         float a = 0;
         _audio.GetOutputData(data, 0);
         foreach (float s in data)
         {
             a += Mathf.Abs(s);
         }
     }
 }