Array index is out of range

Hi When I always play the game it comes up with Array index is out of range? Can someone please help me?

 #pragma strict
 @script RequireComponent( AudioSource )
 var papers : int = 0; // counter of how many papers collected
 var papersToWin : int = 8; // total number of papers in the scene
 var distanceToPaper : float = 5.5; // maximum distance that the raycast will detect
 var sphereRadius : float = 1.0; // the width of the sphere that is being SphereCast
 var paperSound : AudioClip;
 var musicClips : AudioClip[];
 var enemyScript : NPCMovement;
 function Start() 
 {
     if( musicClips.Length < 4 )
     {
         Debug.LogWarning( "Need 4 Music Tracks in the Inspector" );
     }
     
     audio.loop = true;
     audio.clip = musicClips[ 0 ];
     audio.Play();
     
     if ( !enemyScript )
     {
         Debug.LogWarning( "No Enemy Script in the Inspector" );
     }
 }
 function Update() 
 {
     if ( Input.GetMouseButtonDown(0) || Input.GetKeyDown( KeyCode.E ) )
     {
         var hit : RaycastHit;
         
         var rayOrigin : Ray = Camera.main.ScreenPointToRay( Vector3( Screen.width * 0.5, Screen.height * 0.5, 0 ) );
         
         if ( Physics.SphereCast( rayOrigin, sphereRadius, hit, distanceToPaper ) )
         {
             //Debug.Log( "SphereCast Hit : " + hit.collider.gameObject.name );
             //Debug.DrawLine( Camera.main.transform.position, hit.point, Color.red, 1.5 );
             
             if ( hit.collider.gameObject.name == "Paper" )
             {
                 //Debug.Log( "SPHERE hit Paper for sure" );
                 
                 papers += 1;
                 
                 // play paper has been picked up sound
                 audio.PlayClipAtPoint( paperSound, hit.point );
                 
                 // destroy the paper
                 Destroy( hit.collider.gameObject );
                 
                 // tell enemy to follow closer
                 enemyScript.ReduceDistance();
                 
                 // change the music based on the number of papers collected
                 if ( papers == 2 )
                 {
                     audio.Stop();
                     audio.clip = musicClips[ 1 ];
                     audio.Play();
                 }
                 else if ( papers == 4 )
                 {
                     audio.Stop();
                     audio.clip = musicClips[ 2 ];
                     audio.Play();
                 }
                 else if ( papers == 6 )
                 {
                     audio.Stop();
                     audio.clip = musicClips[ 3 ];
                     audio.Play();
                 }
                 else if ( papers == papersToWin )
                 {
                     Debug.Log( "You have collected All Papers !" );
                     
                     // load Win Scene here !!!!
                 }
             }
             
         }
     }
 }