Setting MediaPlayerElement.Source to a relative URI (ms-appx/ms-resource) only works in an app packaged with a Windows Application Packaging Project. If your app does not use a Windows Application Packaging Project, the recommended workaround is to convert the relative ms-appx:/// URI to a fully resolved file:/// URI. Also see the Set the media source and Open local media files sections later in this article.
To set the media source to a media file embedded in the app, initialize a Uri with the path prefixed with ms-appx:///, create a MediaSource with the Uri and then set the Source to the Uri. For example, for a file called video1.mp4 that is in a Videos subfolder, the path would look like: ms-appx:///Videos/video1.mp4
If your app needs access without user interaction to the Music or Video folders, for example, if you are enumerating all the music or video files in the user's collection and displaying them in your app, then you need to declare the Music Library and Video Library capabilities. For more info, see Files and folders in the Music, Pictures, and Videos libraries. 781b155fdc