My BK7-M seems to play some MP3 songs and MIDI files but not all. Why would some play and not all? Does the actual title of the song or Midi file have to follow some kind of protocol? i.e. can a song title have spaces, or numbers at the front of the name, or, underscores in place of text? Here's what I mean... lets say I have a midi file or MP3 song with the title of "In my life" does it matter if the file title on a USB stick is written as this or In-my-life or 1_In-my-life...etc or any combination of text and numbers?