<Window x:Class="TestBackGroundWorker.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ui="clr-namespace:X.UI;assembly=X.UI"  
        xmlns:local="clr-namespace:TestBackGroundWorker"
        Title="MainWindow" Height="681" Width="1000">
    <Window.Resources>
        <Style TargetType="local:BackgroundPlayer">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="local:BackgroundPlayer">
                        <MediaElement x:Name="Media" Stretch="Fill"></MediaElement>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <Grid.Background>
            <VisualBrush>
                <VisualBrush.Visual>                   
                    <local:BackgroundPlayer Source="~/Images/bg.avi"></local:BackgroundPlayer>
                </VisualBrush.Visual>
            </VisualBrush>
        </Grid.Background>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace TestBackGroundWorker
{
  public class BackgroundPlayer : System.Windows.Controls.Control
  {
    static BackgroundPlayer()
    {
      DefaultStyleKeyProperty.OverrideMetadata(typeof(BackgroundPlayer), new FrameworkPropertyMetadata(typeof(BackgroundPlayer)));
    }

    public string Source
    {
      get { return (string)GetValue(SourceProperty); }
      set { SetValue(SourceProperty, value); }
    }
   
    public static readonly DependencyProperty SourceProperty =
      DependencyProperty.Register("Source", typeof(string), typeof(BackgroundPlayer), new FrameworkPropertyMetadata("", (a, b) =>
      {
        BackgroundPlayer bp = a as BackgroundPlayer;
        if (bp.Player != null)
        {
          bp.Player.Source = new Uri(b.NewValue.ToString().Replace("~", AppDomain.CurrentDomain.BaseDirectory), UriKind.RelativeOrAbsolute);
        }        
      }));

    private MediaElement _player;

    public MediaElement Player
    {
      get { return _player; }
      set { _player = value; }
    }

    public override void OnApplyTemplate()
    {
      Player = GetTemplateChild("Media") as MediaElement;
      if (null == Player)
        throw new ArgumentNullException("Media");

      Player.LoadedBehavior = MediaState.Manual;
      Player.MediaEnded += Player_MediaEnded;
      Player.MediaOpened += Player_MediaOpened;
      Player.MediaFailed += Player_MediaFailed;
      Player.Loaded += Player_Loaded;
      if (!string.IsNullOrEmpty(Source))
      {
        Player.Source = new Uri(Source.Replace("~", AppDomain.CurrentDomain.BaseDirectory), UriKind.RelativeOrAbsolute);
        Player.Play();
      }
      base.OnApplyTemplate();
    }

    void Player_Loaded(object sender, RoutedEventArgs e)
    {
    }

    void Player_MediaFailed(object sender, ExceptionRoutedEventArgs e)
    {
    }

    void Player_MediaOpened(object sender, RoutedEventArgs e)
    {
      //Player.Play();
    }

    void Player_MediaEnded(object sender, RoutedEventArgs e)
    {
      Player.Position = TimeSpan.FromMilliseconds(1);
      Player.Play();
    }

  }
}

static BackgroundPlayer()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(BackgroundPlayer), new FrameworkPropertyMetadata(typeof(BackgroundPlayer)));
        }