The `playTrack` function used in MP3ButtonPlayer2 example plays file with name created with printf format "track%03d.mp3". %03d creates a number prefixed with zeros to lenght 3. For example track005.mp3 or track042.mp3 or track999.mp3.

implementation of the playTrack function:

    uint8_t SFEMP3Shield::playTrack(uint8_t trackNo){
    
      //a storage place for track names
      char trackName[] = "track001.mp3";
      uint8_t trackNumber = 1;
    
      //tack the number onto the rest of the filename
      sprintf(trackName, "track%03d.mp3", trackNo);
    
      //play the file
      return playMP3(trackName);
    }