Getting weird effects from audio clip

The first problem is when the audio clip plays. I just want it to play when a unit/units are selected. However its playing every time I release the left mouse button?

 using UnityEngine;
 using System.Collections;
 
 public class Unit : MonoBehaviour
 {
     
     public Projector lightRing;
     public AudioClip YesSir;
     public bool selected = false;
 
     void Start()
     {
         lightRing = GetComponentInChildren<Projector>();
     }
 
 
     private void Update()
     {
         if(renderer.isVisible && Input.GetMouseButton(0))
         {
             Vector3 camPos = Camera.main.WorldToScreenPoint(transform.position);
             camPos.y = CameraOperator.InvertMouseY(camPos.y);
             selected = CameraOperator.selection.Contains(camPos);
         }
 
         if(selected)
             lightRing.enabled = true;
         else
             lightRing.enabled = false;
 
     
         if(renderer.isVisible && lightRing.enabled == false && Input.GetMouseButtonUp(0))
         {
             ConfirmCommand();
         }
     
     }
 
     private void ConfirmCommand()
     {
         audio.PlayOneShot(YesSir);
     }
 }
 

The 2nd issue is the more Units I select the quieter the sound clip plays! (one or two units is fine, 3 - 5 its barely audible and 6 or more and you just cant hear anything)

Why is the volume apparently being shared between the number of units?

Any ideas please guys ???