I tried this with an image file, which works the same as a sound file as far as the uri is concerned because it's just another resource. I used the code below which essentially matches what you have.
new Uri(@"pack://application:,,,/Resources/logo.png")
Make sure that your 'Media' folder is not nested in any other folder. If it is, you need to include that folder as well.
Using .NET Framework 4.0, VS2012.
This link gives a pretty good description of the whole "pack" scheme of things.
EDIT
More research on this topic seems to indicate that what you want to do might not be possible with audio or video files. The excerpt below is taken from the remarks section of this MSDN page.
Although you can declare an instance of this class in Extensible
Application Markup Language (XAML), you cannot load and play its media
without using code. To play media in XAML only, use a MediaElement.
Also, if you declare an instance in XAML, the only practical use is to
fill property element syntax for the Player property.
When distributing media with your application, you cannot use a media
file as a project resource. In your project file, you must instead set
the media type to Content and set CopyToOutputDirectory to
PreserveNewest or Always.
MediaPlayer can be used in two different modes, depending on what is
driving the player: independent mode or clock mode. In independent
mode, the MediaPlayer is analogous to an image and the media opened
through the Open method drives playback. In Clock mode, the
MediaPlayer can be thought of as a target for an animation, and thus
it will have corresponding Timeline and Clock entries in the Timing
tree which controls playback. For more information on media modes, see
the Multimedia Overview.
MediaPlayer is different from a MediaElement in that it is not a
control that can be added directly to the user interface (UI) of an
application. To display media loaded using MediaPlayer, a VideoDrawing
or DrawingContext must be used.
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