...
    await MyWaveBuilder.exampleAppend();
    final Directory tempDir = await getTemporaryDirectory();
    final String tempFile = '${tempDir.absolute.path}/out.wav';
    MyAudioPlayer.play(tempFile);

import 'dart:io';
import 'dart:math';

import 'package:wave_builder/wave_builder.dart';
import 'package:path_provider/path_provider.dart';

class MyWaveBuilder {
  /// Create 1 bar of a 4 random beats
  static Future<void> exampleAppend() async {
    var rng = Random();
    final Directory tempDir = await getTemporaryDirectory();
    final String tempFile = '${tempDir.absolute.path}/out.wav';

    var fileOut = File(tempFile);
    var primary = File('./assets/sounds/primary.wav');
    var secondary = File('./assets/sounds/secondary.wav');
    var silenceType = WaveBuilderSilenceType.BeginningOfLastSample;

    var primaryBytes = await primary.readAsBytes();
    var secondaryBytes = await secondary.readAsBytes();

    await fileOut.create();

    var waveBuilder = WaveBuilder();

    waveBuilder.appendFileContents(primaryBytes);
    waveBuilder.appendSilence(750 - rng.nextInt(500), silenceType);
    waveBuilder.appendFileContents(secondaryBytes);
    waveBuilder.appendSilence(750 - rng.nextInt(500), silenceType);
    waveBuilder.appendFileContents(secondaryBytes);
    waveBuilder.appendSilence(750 - rng.nextInt(500), silenceType);
    waveBuilder.appendFileContents(secondaryBytes);
    waveBuilder.appendSilence(750 - rng.nextInt(500), silenceType);
    await fileOut.writeAsBytes(waveBuilder.fileBytes);
  }
}

import 'dart:io';

import 'package:audioplayers/audioplayers.dart';

class MyAudioPlayer {
  static play(String src) async {
    final player = AudioPlayer();
    player.setReleaseMode(ReleaseMode.release);
    await player.play(DeviceFileSource(src));
  }
}