问使用音频播放器时不会发出任何声音

EN

E/flutter ( 7738): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: Unable to load asset: assets/assets/a.mp3
E/flutter ( 7738): #0      PlatformAssetBundle.load (package:flutter/src/services/asset_bundle.dart:225:7)
E/flutter ( 7738): <asynchronous suspension>
E/flutter ( 7738): #1      AudioCache._fetchAsset (package:audioplayers/audio_cache.dart:75:29)
E/flutter ( 7738): #2      AudioCache.fetchToMemory (package:audioplayers/audio_cache.dart:82:30)
E/flutter ( 7738): <asynchronous suspension>
E/flutter ( 7738): #3      AudioCache.load (package:audioplayers/audio_cache.dart:97:37)
E/flutter ( 7738): #4      AudioCache.getAbsoluteUrl (package:audioplayers/audio_cache.dart:192:23)
E/flutter ( 7738): #5      AudioCache.play (package:audioplayers/audio_cache.dart:122:24)
E/flutter ( 7738): #6      XylophoneApp.build.<anonymous closure> (package:xylophone/main.dart:18:26)
E/flutter ( 7738): #7      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:993:19)
E/flutter ( 7738): #8      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:1111:38)
E/flutter ( 7738): #9      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:183:24)
E/flutter ( 7738): #10     TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:598:11)
E/flutter ( 7738): #11     BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:287:5)
E/flutter ( 7738): #12     BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:222:7)
E/flutter ( 7738): #13     PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:476:9)
E/flutter ( 7738): #14     PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:77:12)
E/flutter ( 7738): #15     PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:122:9)
E/flutter ( 7738): #16     _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:377:8)
E/flutter ( 7738): #17     PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:120:18)
E/flutter ( 7738): #18     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:106:7)
E/flutter ( 7738): #19     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:358:19)
E/flutter ( 7738): #20     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:338:22)
E/flutter ( 7738): #21     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:267:11)
E/flutter ( 7738): #22     GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:295:7)
E/flutter ( 7738): #23     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:240:7)
E/flutter ( 7738): #24     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:213:7)
E/flutter ( 7738): #25     _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter ( 7738): #26     _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter ( 7738): #27     _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)

import 'package:flutter/material.dart';
import 'package:audioplayers/audio_cache.dart';

void main() => runApp(XylophoneApp());

class XylophoneApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: Container(
            child: Center(
              child: FlatButton(
                color: Colors.teal,
                onPressed: () {
                  final player = AudioCache();
                  player.play('a.mp3');
                },
                child: Text('Click button'),
              ),
            ),
          ),
        ),
      ),
    );
  }
}