Comment 13 for bug 1245495

The problem is that MPRIS provides no way for the sound menu to find out when an application stops exporting player functionality. So if you've had the plugin enabled once, the sound menu will continue to display rhythmbox even after you've disabled the plugin again.

You can remove media players from the sound menu manually by modifying the com.canonical.indicator.sound interested-media-players gsettings key in dconf-editor.