MP3test.ino

#include <SoftwareSerial.h>
#include "RedMP3.h"

#define MP3_RX 33//RX of Serial MP3 module connect to D7 of Arduino
#define MP3_TX 36//TX to D8, note that D8 can not be used as RX on Mega2560, you should modify this if you donot use Arduino UNO
MP3 mp3(MP3_RX, MP3_TX);

int8_t volume = 0x1a;//0~0x1e (30 adjustable level)
int8_t folderName = 0x01;//folder name must be 01 02 03 04 ...
int8_t fileName = 0x002; // prefix of file name must be 001xxx 002xxx 003xxx 004xxx ...
void setup()
{
  delay(500);//Requires 500ms to wait for the MP3 module to initialize  
  mp3.setVolume(volume);
  delay(50);//you should wait for >=50ms between two commands
  mp3.playWithFileName(folderName,fileName);
  delay(50);
}

void loop()
{
  
}