RemoveStoryboard, classe

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  Name="RootPage">
  <StackPanel>

    <!-- The MediaElement control plays the sound. -->
    <MediaElement Name="myMediaElement" />

    <Button Name="BeginButton">Begin</Button>

    <StackPanel.Triggers>

      <!-- This trigger causes the Storyboard to start (the music start) when you click the
           "BeginButton" button. -->
      <EventTrigger RoutedEvent="Button.Click" SourceName="BeginButton">
        <EventTrigger.Actions>
          <BeginStoryboard Name="myBeginStoryboard">
            <Storyboard>
              <MediaTimeline Source="C:\WINDOWS\Media\town.mid" Storyboard.TargetName="myMediaElement"  
               RepeatBehavior="Forever" />
            </Storyboard>
          </BeginStoryboard>
        </EventTrigger.Actions>
      </EventTrigger>

      <!-- When this page unloads, like when you navigate away from the page, this
           trigger causes the Storyboard to be removed and the clock to stop. If 
           you come back to this page and click the button to start the Storyboard,
           the music starts from the beginning. Alternatively, if you don't unload 
           the Storyboard when you leave the page, the Storyboard does not stop while 
           you're gone and when you navigate back to the page you hear the music  
           continuing as though you had never left. -->
      <EventTrigger RoutedEvent="Page.Unloaded" >
        <EventTrigger.Actions>
          <RemoveStoryboard BeginStoryboardName="myBeginStoryboard" />
        </EventTrigger.Actions>
      </EventTrigger>
    </StackPanel.Triggers>
  </StackPanel>
</Page>