This is something I've wanted to see for a very long time. A managed mp3 player with a bunch of visual playback features. Ahura Mazda created the Saluse Media Kit for Silverlight and it is awesome! There is a demo over at http://prefix.teddywino.com/post/SilverlightMediaKitLiveDemo.aspx.
Ahura based his MediaStreamSource off a MSDN example called the ManagedMediaHelpers.