Python code for AtomS3U, working to play first track in root directory

import os, sys, io
import M5
from M5 import *
import time
from hardware import UART

uart1 = None

str2 = None
name = None
number = None
ok = None
command = None
msg = None
pm = None
bytes2 = None
reply = None

# Describe this function...
def WriteMP3Player(str2):
  global name, number, ok, command, msg, pm, bytes2, reply, uart1
  ok = False
  print(str2)
  uart1.write((str2.encode()))
  return True

# Describe this function...
def getReply():
  global str2, name, number, ok, command, msg, pm, bytes2, reply, uart1
  time.sleep(1)
  msg = bytearray(100)
  while uart1.any():
    bytes2 = uart1.read()
    msg = msg + bytes2
    time.sleep_ms(100)
  return (msg.decode()).strip()

# Describe this function...
def prettyPrintReply(str2):
  global name, number, ok, command, msg, pm, bytes2, reply, uart1
  print(str((len(str2))))
  print(str2)
  if str2 == 'ERR:1':
    print('error 1 received')

# Describe this function...
def playByName(name):
  global str2, number, ok, command, msg, pm, bytes2, reply, uart1
  command = 'AT+SPLAY=sd0,'
  command = command + name
  ok = WriteMP3Player(command)
  reply = getReply()

# Describe this function...
def playByNumber(number):
  global str2, name, ok, command, msg, pm, bytes2, reply, uart1
  command = 'AT+PLAY=sd0,'
  command = command + (str(number))
  ok = WriteMP3Player(command)
  reply = getReply()


def setup():
  global uart1, ok, command, msg, str2, pm, bytes2, name, reply, number

  M5.begin()
  uart1 = UART(1, baudrate=115200, bits=8, parity=None, stop=1, tx=2, rx=1)
  pm = 0
  msg = ''


def loop():
  global uart1, ok, command, msg, str2, pm, bytes2, name, reply, number
  M5.update()
  if pm == 0:
    ok = WriteMP3Player('AT+BAUD?')
    reply = getReply()
    print(reply)
    pm = pm + 1
  if pm == 1:
    ok = WriteMP3Player('AT+GVER?')
    reply = getReply()
    print(reply)
    pm = pm + 1
  if pm == 2:
    ok = WriteMP3Player('AT+GCFGVER?')
    reply = getReply()
    print(reply)
    prettyPrintReply(reply)
    pm = pm + 1
  if pm == 3:
    playByNumber(3)
    reply = getReply()
    print(reply)
    prettyPrintReply(reply)
    pm = pm + 1
  if pm == 4:
    ok = WriteMP3Player('AT+REPEATMODE=4')
    reply = getReply()
    print(reply)
    prettyPrintReply(reply)
    pm = pm + 1
  if pm == 5:
    ok = WriteMP3Player('AT+VOL=3')
    reply = getReply()
    print(reply)
    prettyPrintReply(reply)
    pm = pm + 1


if __name__ == '__main__':
  try:
    setup()
    while True:
      loop()
  except (Exception, KeyboardInterrupt) as e:
    try:
      from utility import print_error_msg
      print_error_msg(e)
    except ImportError:
      print("please update to latest firmware")

Arduino test sketch

https://static-cdn.m5stack.com/resource/arduino/package_m5stack_index.json

#include <M5Unified.h>

// Hardware Serial 1 configuration
HardwareSerial uart1(1); 
const int RX_PIN = 1;
const int TX_PIN = 2;

// Global Variables
bool ok = false;
String str2 = "";
String name = "";
int number = 0;
String command = "";
String msg = "";
int pm = 0;
String reply = "";

// Forward declarations of functions
bool WriteMP3Player(String str2);
String getReply();
void prettyPrintReply(String str2);
void playByName(String name);
void playByNumber(int number);

void setup() {
  // Initialize the M5 device
  auto cfg = M5.config();
  M5.begin(cfg);
  
  // Initialize standard Serial for debug printing to PC
  Serial.begin(115200);
  
  // Initialize UART1 for the MP3 player (Baud rate, Config, RX Pin, TX Pin)
  uart1.begin(115200, SERIAL_8N1, RX_PIN, TX_PIN);
  
  pm = 0;
  msg = "";
}

void loop() {
  M5.update();
  
  if (pm == 0) {
    ok = WriteMP3Player("AT+BAUD?");
    reply = getReply();
    Serial.println(reply);
    pm = pm + 1;
  }
  else if (pm == 1) {
    ok = WriteMP3Player("AT+GVER?");
    reply = getReply();
    Serial.println(reply);
    pm = pm + 1;
  }
  else if (pm == 2) {
    ok = WriteMP3Player("AT+GCFGVER?");
    reply = getReply();
    Serial.println(reply);
    prettyPrintReply(reply);
    pm = pm + 1;
  }
  else if (pm == 3) {
    playByNumber(3);
    reply = getReply();
    Serial.println(reply);
    prettyPrintReply(reply);
    pm = pm + 1;
  }
  else if (pm == 4) {
    ok = WriteMP3Player("AT+REPEATMODE=4");
    reply = getReply();
    Serial.println(reply);
    prettyPrintReply(reply);
    pm = pm + 1;
  }
  else if (pm == 5) {
    ok = WriteMP3Player("AT+VOL=3");
    reply = getReply();
    Serial.println(reply);
    prettyPrintReply(reply);
    pm = pm + 1;
  }
}

// Function to write data to the MP3 Player over UART
bool WriteMP3Player(String str2) {
  ok = false;
  Serial.println(str2); // Print to computer console for debugging
  uart1.print(str2);    // Send AT command to MP3 Player
  return true;
}

// Function to gather responses from the MP3 Player
String getReply() {
  delay(1000); // Wait 1 second for device to start processing
  msg = "";
  
  while (uart1.available()) {
    char c = uart1.read();
    msg += c;
    delay(10); // Short delay to let next byte arrive in buffer
  }
  
  msg.trim(); // Removes whitespace/newlines from ends (equivalent to .strip())
  return msg;
}

// Function to print details about the reply
void prettyPrintReply(String str2) {
  Serial.println(str2.length());
  Serial.println(str2);
  if (str2 == "ERR:1") {
    Serial.println("error 1 received");
  }
}

// Function to play a track by its string name
void playByName(String name) {
  command = "AT+SPLAY=sd0,";
  command = command + name;
  ok = WriteMP3Player(command);
  reply = getReply();
}

// Function to play a track by its index number
void playByNumber(int number) {
  command = "AT+PLAY=sd0,";
  command = command + String(number);
  ok = WriteMP3Player(command);
  reply = getReply();
}