Nick Gravelyn talks about the changes in MediaPlayer, the interface for playing music on Windows Phone, that can affect your applications.
Source: Nick Gravelyn's Blog
The short is that you are no longer allowed to set the Volume or IsMuted properties when the active media source is the Zune media library and any time you switch between library music and title music, you must reset any properties on MediaPlayer you care about. These changes were made to ensure applications didn’t alter the user’s music playback experience in a way that the native Zune media player cannot reverse (there is no UI in the Zune media player for music-specific volume or a mute control).