How to play a custom sound in Flutter?

Simple solution for playing a file already defined in assets is using AudioCache.
Library: https://pub.dartlang.org/packages/audioplayers.
More about AudioCache
After adding library to pubspec.yaml, import required class:

import 'package:audioplayers/audio_cache.dart';

add an asset in the same file and place the file with sound to assets folder (if you don’t have this folder, create it)

assets:
- assets/sound_alarm.mp3

then add this code:

static AudioCache player = new AudioCache();
const alarmAudioPath = "sound_alarm.mp3";
player.play(alarmAudioPath);

An example here