If you want to set an MP3 song as a Ringtone on your Galaxy Ace or Ace 2.  The best route to go would be to download Rocket Player from the Google Play Store and install it.  See below screenshot on how I selected an MP3 from my music library and hit the menu button to show me the option to set it as ringtone. set as mp3 ringtone galaxy ace 2 rocket player select ringtone mp3 galaxy ace 2