MediaElement interrface, where is it?

Hi there,

LoadedBehaviour is put on "Play" value, why don't appear in my view when I run my app??

XAML:

<MediaElement HorizontalAlignment="Left" Height="100" Margin="311,276,0,0" LoadedBehavior="Play" VerticalAlignment="Top" Width="100"/>

As a result, I don't see anything.. what am I missing here???

Design time:

Runtime:

Thanks in advance for any hint/input,

Primary platform is Windows 7 Ultimate 64 bit along with VS 2012/Sql2k8 for WPF/SilverLight projects.