Up until now I thought that changing the system volume from your app is impossible, until recently a new app called Quite Hours (http://www.windowsphone.com/en-us/st...1-e55046ee5a2a) actually did just that in a very neat way.
Does anyone please know how to do that programmatically? I tried using the MediaElement or the xna MediaPlayer and the backgroundAudioPlayer and nothing worked. Any help would be appreciated! Thank you!