C# Windows Phone 8.1 BackgroundAudioPlayer play new music

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C#

Windows-Phone-8

Hi,
I'v been created a Background Audio Player in a Universal project for Windows Phone 8.1 by this sample>
https://code.msdn.microsoft.com/windowsapps/BackgroundAudio-63bbc319[^]

everything works fine, and I can play with URL. I changed BackgroundAudioTask to play URL by user in the RunTime. at first time User set a new music everything works fine , when user tries to set new music URL, old music will be Pause and when press on the PlayButton in application, it Plays old music on the last position!. didn't set new music.

how can I set new URL to play? (I don't want use playlist)

thanks