main.dart

import 'package:audioplayers/audio_cache.dart';

class _MainPageState extends State<MainPage> with SingleTickerProviderStateMixin {
  AudioCache player = new AudioCache(); //<-- use AudioCache
  ...
    
  void _onTap(Note note) {
    ...
    _playNote(note); //<-- play a note on tap
    setState(() {
      note.state = NoteState.tapped;
      ++points;
    });
  }

  _playNote(Note note) {
    switch (note.line) { //<-- choose a sound depending on the note's line
      case 0:
        player.play('a.wav');
        return;
      case 1:
        player.play('c.wav');
        return;
      case 2:
        player.play('e.wav');
        return;
      case 3:
        player.play('f.wav');
        return;
    }
  }
}