If the music is stored on MicroSD or google music (streamed) even though the widget looks inconspicuous in itself, you must remember it still means the application is running in the background (in order for the widget to function - the app the widget is for must be running/accessible) and therefore if the app is accessing music from either of those sources then media server background task will be active - regardless of whether your listening to the music or not.
However not all Android OS's are created equal and it seems according to XDA the media server issue is pointing to MicroSD access for the majority of users with the problem. It also appears to be more Touchwiz based as S3 & Note 2 users have all reported similar background service running in background.
Also....
It also doesn't have to be music, if you have your phones camera photos or wallpapers saved on the MicroSD and you have one of those wallpapers / camera photos setup as your desktop / lock screen wallpaper then you will see this accessing if Media Server too as it basically is constantly accessing the MicroSD to display the wallpaper....
Like I say best way to see if it is the MicroSD accessing that is your own Media Server background task user - is to remove the microSD, if usage drops then there is the culprit.
And if it is the culprit and you want to stop the continuous access of Media Server, either turn off music widget, and move any photo you want as desktop wallpapers to the on-board storage, so that the phone doesn't need to continuously access the MicroSD.
