This is the source code of the Flutter Challenges series post: https://fun4code.com/10flutterchallanges-part3/

audio_service.dart

import 'package:audioplayers/audioplayers.dart';

abstract class AudioService {
  static AudioCache audioCache = AudioCache();
  static AudioPlayer audioPlayer = AudioPlayer(
    playerId: 'my_unique_playerId',
    mode: PlayerMode.LOW_LATENCY,
  );

  static void play(String filename) async {
    audioPlayer = await audioCache.play(filename);
  }

  static void stop() async => await audioPlayer.stop();
}

lifecycle_service.dart

import 'package:flutter/material.dart';
import 'package:samples/audio_service/audio_service.dart';

class LifeCycleService extends StatefulWidget {
  final Widget child;

  const LifeCycleService({Key? key, required this.child}) : super(key: key);

  @override
  _LifeCycleServicerState createState() => _LifeCycleServiceState();
}

class _LifeCycleServiceState extends State<LifeCycleService>
    with WidgetsBindingObserver {
  @override
  void initState() {
    WidgetsBinding.instance!.addObserver(this);
    super.initState();
  }

  @override
  void dispose() {
    WidgetsBinding.instance!.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    switch (state) {
      case AppLifecycleState.resumed:
        AudioService.play('music/track_01.mp3');
        break;
      case AppLifecycleState.inactive:
      case AppLifecycleState.paused:
      case AppLifecycleState.detached:
        AudioService.stop();
        break;
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: widget.child,
    );
  }
}

main.dart

import 'package:flutter/material.dart';
import 'package:samples/audio_service/audio_service.dart';
import 'package:samples/lifecycle_widget.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    AudioService.play('music/track_01.mp3');
    return const LifeCycleService(
      child: MaterialApp(
        home: Scaffold(
          body: Text('App'),
        ),
      ),
    );
  }
}