There is no 'AudioSource' attached to the "Cube" game object, but a script is trying to access it.

I create a primitive cube and add an AudioSource to it and when I access it it works fine...

 void Start()
     {
         this.gameObject.AddComponent<AudioSource>();
         Au = this.GetComponent<AudioSource>();
     }

but I have a condition that the audio plays only if the UI is not active .

 if (!EG.UI.activeSelf)
             {
                 Au = this.GetComponent<AudioSource>();
                 Au.volume = Mathf.Clamp01(collision.relativeVelocity.magnitude / 45);
                 Au.pitch = .25f;
                 Au.Play();
             }

Then when the UI is active it gives me this error:

 MissingComponentException: There is no 'AudioSource' attached to the "Cube" game object, but a script is trying to access it.
 You probably need to add a AudioSource to the game object "Cube". Or your script needs to check if the component is attached before using it.
 Piece.OnCollisionEnter (UnityEngine.Collision collision) (at Assets/Scripts/Piece.cs:119)

but in the scene the AudioSource is in the cube components...

Any idea why this happens?