Volume Slider

I created a slider that is supposed to edit volume. How can I make it so the volume changes with the slider. I tried some things and it didn't work. I keep getting a null reference exception that the object reference is not set to an instance of an object.

Here is one of the functions.

 function Update () 
 {
     if (!GameObject.FindGameObjectWithTag("MM") && CreateMusic == true) {
         var musicManager = Instantiate (musicPrefab, transform.position, Quaternion.identity);
         musicManager.name = musicPrefab.name;
         if(musicManager.GetComponent.<AudioSource>().enabled == false) {
             musicManager.GetComponent.<AudioSource>().enabled = true;
         }    
         musicMenu = musicManager;
     }
     musicManager.GetComponent.<AudioSource>().volume = MainMenu.Volume;
 }

another function that does the same thing but for a different level.

 function Update () 
 {
     if (!GameObject.FindGameObjectWithTag("MM") && CreateMusic == true) {
         var mManager = Instantiate (musicPrefab, transform.position, Quaternion.identity);
         mManager.name = musicPrefab.name;
         if(mManager.GetComponent.<AudioSource>().enabled == false) {
             mManager.GetComponent.<AudioSource>().enabled = true;
         }    
         MusicManager = mManager;
     }
     mManager.GetComponent.<AudioSource>().volume = MainMenu.Volume;
 }

Here is the script that handles volume.

 function SetGameVolume (vol : float) {
     Volume = vol; 
 }

What is supposed to happen is that the Volume var is set to the volume of the audio sources. But it is not working. I want the volume bar to work in multiple levels. How can I do this?