PlayAudioInLaunchWP.cs

public MainPage()
{
  InitializeComponent();
  startBackgroundMusic();
}

private void startBackgroundMusic()
{
  this.Dispatcher.BeginInvoke(() =>
  {
    StreamResourceInfo info = Application.GetResourceStream(
      new Uri("Assets/background.wav", UriKind.Relative));
    backgroundMusic = SoundEffect.FromStream(info.Stream);
    SoundEffectInstance instance = backgroundMusic.CreateInstance();
    instance.IsLooped = true;
    instance.Play();
                       
  });
}