yeah you can't do it from settings but how os x treats different types of files and their associated applications. if you want to change the player, grab something like VLC. then on a .avi for a example, get info for that file, then in the info window there is a "Open With:" change it to VLC and then hit the change all which should in theory change everything with the .avi extension to play in VLC rather then QT by default.