Windows 8.1音频流没有在后台播放

我正在制作一个可以在后台播放音频流的Windows 8.1应用程序。但是它不会在后台播放音频流例如如果应用程序被最小化。当应用处于焦点时，它可以很好地播放音频流。我希望有人能帮上忙。谢谢。

MainPage.xaml:

<Grid Name="mainGrid" Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
    <TextBlock TextWrapping="Wrap" Text="Radio Limfjord" VerticalAlignment="Top" Height="116" FontSize="100" Margin="360,10,360,0"/>
    <MediaElement Visibility="Visible" Name="mediaplayer" AudioCategory="BackgroundCapableMedia" AutoPlay="True" RealTimePlayback="True" AudioDeviceType="Multimedia" HorizontalAlignment="Right" Width="1218" Margin="0,366,63,177" AreTransportControlsEnabled="True" CurrentStateChanged="mediaplayer_CurrentStateChanged" />
    <Button Name="cmdPlay" Content="Afspil" Margin="566,627,0,41" Click="cmdPlay_Click" Width="100" Height="100"/>
    <Button Name="cmdStop" Content="Stop" Margin="714,626,0,41" Click="cmdStop_Click" Width="100" Height="100"/>
</Grid>

MainPage.xaml.cs:

public sealed partial class MainPage : Page
{
    SystemMediaTransportControls systemControls;
    public MainPage()
    {
        this.InitializeComponent();
    }
    void InitializeTransportControls()
    {
        // Hook up app to system transport controls.
        systemControls = SystemMediaTransportControls.GetForCurrentView();
        systemControls.ButtonPressed += SystemControls_ButtonPressed;
        // Register to handle the following system transpot control buttons.
        systemControls.IsPlayEnabled = true;
        systemControls.IsPauseEnabled = true;
    }
    void MusicPlayer_CurrentStateChanged(object sender, RoutedEventArgs e)
    {
        switch (mediaplayer.CurrentState)
        {
            case MediaElementState.Playing:
                systemControls.PlaybackStatus = MediaPlaybackStatus.Playing;
                break;
            case MediaElementState.Paused:
                systemControls.PlaybackStatus = MediaPlaybackStatus.Paused;
                break;
            case MediaElementState.Stopped:
                systemControls.PlaybackStatus = MediaPlaybackStatus.Stopped;
                break;
            case MediaElementState.Closed:
                systemControls.PlaybackStatus = MediaPlaybackStatus.Closed;
                break;
            default:
                break;
        }
    }
    void SystemControls_ButtonPressed(SystemMediaTransportControls sender,
SystemMediaTransportControlsButtonPressedEventArgs args)
    {
        switch (args.Button)
        {
            case SystemMediaTransportControlsButton.Play:
                PlayMedia();
                break;
            case SystemMediaTransportControlsButton.Pause:
                PauseMedia();
                break;
            default:
                break;
        }
    }
    async void PlayMedia()
    {
        await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
        {
            mediaplayer.Play();
        });
    }
    async void PauseMedia()
    {
        await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
        {
            mediaplayer.Pause();
        });
    }
    private void cmdPlay_Click(object sender, RoutedEventArgs e)
    {
        mediaplayer.Source = new Uri("http://media.wlmm.dk/limfjord", UriKind.Absolute);
        mediaplayer.Play();
    }
    private void cmdStop_Click(object sender, RoutedEventArgs e)
    {
        mediaplayer.Stop();
    }
    private void mediaplayer_CurrentStateChanged(object sender, RoutedEventArgs e)
    {
    }
}