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);
}

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);
}

change it to:

uint8_t SFEMP3Shield::playTrack(uint8_t trackNo){

  //a storage place for track names
  char trackName[13];
  int trackNumber = trackNo;

  //tack the number onto the rest of the filename
  sprintf(trackName, "track%03d.mp3", trackNumber);

  //play the file
  return playMP3(trackName);
}