Den Delimarsky demonstrates how to play existing media content on WP7 using the MediaElement control.
Source: DZone
Ultimately, this article started from a simple question - how is it possible to play a media file on Windows Phone 7? There are two ways to play existing media content. The developer has to either use the system player (given that the media content is stored in the media library), or use the MediaElement control.
MediaElement might be a bit tricky, but it's not that hard as it seems.