I'll go on record as saying I recommended OOMA, not magic jack :)
Ive never used media player but i assume your using the players media management. im guessing theres a drill down / file explorer in settings that you set for the folders where you keep you music and some how these have changed. I think when you first launch media player a wizard opens to scan for media files surely theres away to rerun the wizard. sorry cant be more helpful. good luck
+OOMA coupled with Google voice