<Grid>
        <MediaElement Name="Player" Width="500" Height="500">
            
        </MediaElement>
        <Button Name="Button" Margin="0,177,0,0">
            
        </Button>      
    </Grid>

         public MainWindow()
        {
            InitializeComponent();

            Player.LoadedBehavior = MediaState.Manual;
            Player.UnloadedBehavior = MediaState.Manual;
          
            Button.Click += new RoutedEventHandler(Button_Click);
        }
      
        void Button_Click(object sender, RoutedEventArgs e)
        {
            
            if (Player.Source == null)
                Player.Source = new Uri("filename");
            else
                Player.Position = TimeSpan.FromSeconds(0);                                

            Player.Play();
        }