Example of MP3 Play for M5Stack Core2

PlayMP3FromSDToDAC.ino

#pragma mark - Depend ESP8266Audio and ESP8266_Spiram libraries
/* 
cd ~/Arduino/libraries
git clone https://github.com/earlephilhower/ESP8266Audio
git clone https://github.com/Gianbacchio/ESP8266_Spiram
*/


#include <M5Core2.h>
#include <driver/i2s.h>
#include <WiFi.h>
#include "AudioFileSourceSD.h"
#include "AudioFileSourceID3.h"
#include "AudioGeneratorMP3.h"
#include "AudioOutputI2S.h"


AudioGeneratorMP3 *mp3;
AudioFileSourceSD *file;
AudioOutputI2S *out;
AudioFileSourceID3 *id3;

#define OUTPUT_GAIN 10

void setup()
{
  M5.begin();
  M5.Axp.SetSpkEnable(true);

  WiFi.mode(WIFI_OFF); 
  delay(500);
  
  M5.Lcd.setTextFont(2);
  M5.Lcd.printf("Sample MP3 playback begins...\n");
  Serial.printf("Sample MP3 playback begins...\n");

  // pno_cs from https://ccrma.stanford.edu/~jos/pasp/Sound_Examples.html
  file = new AudioFileSourceSD("/mp3/chikichiki.mp3");
  id3 = new AudioFileSourceID3(file);
  out = new AudioOutputI2S(0, 0); // Output to builtInDAC
  out->SetPinout(12, 0, 2);
  out->SetOutputModeMono(true);
  out->SetGain((float)OUTPUT_GAIN/100.0);
  mp3 = new AudioGeneratorMP3();
  mp3->begin(id3, out);
}

void loop()
{
  if (mp3->isRunning()) {
    if (!mp3->loop()) mp3->stop();
  } else {
    Serial.printf("MP3 done\n");
    delay(1000);
  }
}

Perhaps you also need to add the following line.

SPIFFS.begin();