If you want to create a Live TV player with Silverlight, perhaps are you going to face with some troubles if you want to switch from a live stream to another. The problem comes if you try to use Stop and Play methods. Instead, you have to use the AutoPlay property

Here is a sample :

...
    media = rootElement.findName("mediaElement");   
    media.addEventListener("MediaOpened", Silverlight.createDelegate(this, this.handleMediaOpened));
...

setMediaElementSource : function(sender, url) {
    this.startWaiting(sender); // start an animation
    media.AutoPlay="False"
    media.Source = url ;
   media.AutoPlay="True"
},

handleMediaOpened: function(sender, eventArgs)
{
    this.stopWaiting(sender); // stop the animation
},

startWaiting : function(sender)
{
    sender.findName("StoryboardWaiting").begin();
},

stopWaiting : function(sender)
{
    sender.findName("StoryboardWaiting").stop();
}