title: Background music overlap when back to creator viewController
body: <p>I have a game which consists of 3 view controllers:</p>
  
  <ul>
  <li>viewController</li>
  <li>settingViewController</li>
  <li>GameViewController</li>
  </ul>
  
  <p>I have set the background music and played it in the viewController class</p>
  
  <pre><code>var backgroundMusic : AVAudioPlayer!
  func setUpSounds(){
          //button sound
          if let buttonSoundPath = NSBundle.mainBundle().pathForResource("buttonClick", ofType: "mp3") {
              let buttonSoundURL = NSURL(fileURLWithPath: buttonSoundPath)
              do {
                  try buttonSound = AVAudioPlayer(contentsOfURL: buttonSoundURL)
              }catch {
                  print("could not setup button sound")
              }
              buttonSound.volume = 0.5
          }
          //background sound
          if let backgroundMusicPath = NSBundle.mainBundle().pathForResource("BackgroundMusic", ofType: "mp3") {
              let backgroundMusicURL = NSURL(fileURLWithPath: backgroundMusicPath)
              do {
                  try backgroundMusic = AVAudioPlayer(contentsOfURL: backgroundMusicURL)
              }catch {
                  print("could not setup background music")
              }
              backgroundMusic.volume = 0.2
              /*
              set any negative integer value to loop the sound
              indefinitely until you call the stop method
              */
              backgroundMusic.numberOfLoops = -1
          }
      }
  
  
  override func viewDidLoad() {
          super.viewDidLoad()
          self.setUpSounds()
          self.playBackgroundSound()
          // Do any additional setup after loading the view, typically from a nib.
      }
  </code></pre>
  
  <p>when I move to the settingViewController then back again to the viewController the sound replay and overlap the old played music. </p>
  
  <p>What is the solution of this problem?</p>