Trying to make a volume slider bar, Part 2

I'm trying to make a slider bar that controls the volume of the background music, and another that controls the volume of the sound effects. I'm working on the music first. I have the slider bar made, but I don't know how to code it to control the music. The background music is an audio file being played from an Audio Source on a gameobject tagged GameController. Here is my code so far.

 using UnityEngine;
 using System.Collections;
  
 public class Menu : MonoBehaviour 
 {
  
     public bool gameIsPaused = false;
     public bool buttons = true;
 
     public GameObject gameController;
     public GameObject musicVolume;
 
     private float hSliderValue = 0.0f;
 
     void Start() 
     {
         gameController = GameObject.FindGameObjectWithTag("GameController");
         musicVolume = gameController.GetComponent<AudioSource>();
     }
  
     void Update()
     {
         if(Input.GetKeyUp(KeyCode.P))
         {
             if(gameIsPaused)
             {
                 gameIsPaused = false;
                 Time.timeScale = 1.0f;
             } else 
             {
                 gameIsPaused = true;
                     Time.timeScale = 0.0f;
             }
         }
 
     }
  
     void OnGUI()
     {
         if(gameIsPaused) 
         {
             if(buttons)
             {
                 if(GUI.Button(new Rect(Screen.width / 2 - 100, Screen.height / 2 - 75, 200, 50), "Resume"))
                 {
                     gameIsPaused = false;
                     Time.timeScale = 1.0f;
                 }
 
                 if(GUI.Button(new Rect(Screen.width / 2 - 100, Screen.height / 2 - -25, 200, 50), "Settings"))
                 {
                     buttons = false;
                 }
                 if(GUI.Button(new Rect(Screen.width / 2 - 100, Screen.height / 2 - -125, 200, 50), "Main Menu"))
                 {
                     Application.LoadLevel("MainMenu2");
                 }
 
             }
 
             if(buttons == false) 
             {
                 if(GUI.Button(new Rect(Screen.width / 2 - 100, Screen.height / 2 - -25, 200, 50), "Back"))
                 {
                     buttons = true;
                 }
                 hSliderValue = GUI.HorizontalSlider (new Rect (Screen.width / 2 - 60, Screen.height / 2 - -100, 200, 50), hSliderValue, 0.0f, 10.0f);
                 GUI.Label (new Rect (Screen.width / 2 - 100, Screen.height / 2 - -100, 200, 50), "Music");
             }
         }
     }
 }

I thought I could reference the Audio Source by using GetComponent from the GameController gameobject. But I am really rather stuck right now. This code gives me this error

 Assets/Scripts/Menu.cs(18,17): error CS0029: Cannot implicitly convert type `UnityEngine.AudioSource' to `UnityEngine.GameObject'

Thanks!