This sample code is for Audio Dorbell Tutorial.

audio_doorbell.ino

#pragma mark - Depend ESP8266Audio and ESP8266_Spiram libraries
#include <M5Stack.h>
#include <WiFi.h>
#include "AudioFileSourceSD.h"
#include "AudioFileSourceID3.h"
#include "AudioGeneratorMP3.h"
#include "AudioOutputI2S.h"

AudioGeneratorMP3 *mp3;
AudioFileSourceSD *file;
AudioOutputI2S *out;
AudioFileSourceID3 *id3;
bool playing = false;
bool curr = false;
bool prev = false;

void setup() {
  M5.begin();
  M5.Power.begin();

  pinMode(36, INPUT);
  Serial.begin(9600);
}

void loop() {
  curr = digitalRead(36);
  if (curr != prev) {
    if (curr == true) {
      M5.Lcd.setTextColor(WHITE);
      M5.Lcd.setCursor(30, 70);
      M5.Lcd.setTextSize(10);
      M5.Lcd.printf("Hello!Welcome");
      play();
    }
    prev = curr;
  }

  if (playing) {
    if (mp3->isRunning()) {
      if (!mp3->loop()) {
        mp3->stop();
        playing = false;
        M5.Lcd.fillScreen(BLACK);
        delay(500);
      }
    } else {
      delay(1000);
    }
  }
  M5.update();
}

bool play() {
  if (file) {
    delete file;
    file = NULL;
  }
  if (mp3) {
    delete mp3;
    mp3 = NULL;
  }
  if (out) {
    delete out;
    out = NULL;
  }
  file = new AudioFileSourceSD("/Hello.mp3");
  id3 = new AudioFileSourceID3(file);
  out = new AudioOutputI2S(0, 1);
  out->SetOutputModeMono(true);
  mp3 = new AudioGeneratorMP3();
  mp3->begin(id3, out);
  playing = true;
  return true;
}