Repository: flutter
Issue: SystemSound.play should not lower volume of music playback on iOS
Labels: platform-ios,framework,engine,has reproducible steps,P2,found in release: 3.7,found in release: 3.8,team-ios,triaged-ios

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Create an IconButton or BottomNavigationBar (I believe this is general behavior but I've specifically tested those widgets.)
2. Tap one while playing music on iOS. The music plays quieter for the duration of the tap sound effect.

**Expected results:** <!-- what did you want to see? -->
Music playback is unaffected.

**Actual results:** <!-- what did you see? -->
The music plays quieter for the duration of the tap sound effect.

<details>
  <summary>Logs</summary>

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
[✓] Flutter (Channel stable, v1.17.1, on Mac OS X 10.15.4 19E287, locale en-US)
    • Flutter version 1.17.1 at [REDACTED]
    • Framework revision f7a6a7906b (2 weeks ago), 2020-05-12 18:39:00 -0700
    • Engine revision 6bc433c6b6
    • Dart version 2.8.2

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.0)
    • Android SDK at [REDACTED]
    • Platform android-29, build-tools 29.0.0
    • Java binary at: /Library/Java/JavaVirtualMachines/jdk1.8.0_202.jdk/Contents/Home/jre/bin/java
    • Java version Java(TM) SE Runtime Environment (build 1.8.0_202-ea-b03)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.5)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.5, Build version 11E608c
    • CocoaPods version 1.8.4

[!] Android Studio (not installed)
    • Android Studio not found; download from https://developer.android.com/studio/index.html
      (or visit https://flutter.dev/docs/get-started/install/macos#android-setup for detailed instructions).

[!] IntelliJ IDEA Ultimate Edition (version 2020.1.1)
    • IntelliJ at /Applications/IntelliJ IDEA.app
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • For information about installing plugins, see
      https://flutter.dev/intellij-setup/#installing-the-plugins

[!] VS Code (version 1.44.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    ✗ Flutter extension not installed; install from
      https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[✓] Connected device (2 available)
    • Pixel 2      • [REDACTED]        • android-arm64 • Android 10 (API 29)
    • iPhone • [REDACTED] • ios           • iOS 13.5
```

</details>

## Sample Code
See: https://api.flutter.dev/flutter/material/BottomNavigationBar-class.html  
(The issue can be seen when tapping items in the BottomNavigationBar)

```dart
// Flutter code sample for BottomNavigationBar

// This example shows a [BottomNavigationBar] as it is used within a [Scaffold]
// widget. The [BottomNavigationBar] has three [BottomNavigationBarItem]
// widgets and the [currentIndex] is set to index 0. The selected item is
// amber. The `_onItemTapped` function changes the selected item's index
// and displays a corresponding message in the center of the [Scaffold].
//
// ![A scaffold with a bottom navigation bar containing three bottom navigation
// bar items. The first one is selected.](https://flutter.github.io/assets-for-api-docs/assets/material/bottom_navigation_bar.png)

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

/// This Widget is the main application widget.
class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  int _selectedIndex = 0;
  static const TextStyle optionStyle =
      TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
  static const List<Widget> _widgetOptions = <Widget>[
    Text(
      'Index 0: Home',
      style: optionStyle,
    ),
    Text(
      'Index 1: Business',
      style: optionStyle,
    ),
    Text(
      'Index 2: School',
      style: optionStyle,
    ),
  ];

  void _onItemTapped(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('BottomNavigationBar Sample'),
      ),
      body: Center(
        child: _widgetOptions.elementAt(_selectedIndex),
      ),
      bottomNavigationBar: BottomNavigationBar(
        items: const <BottomNavigationBarItem>[
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            title: Text('Home'),
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.business),
            title: Text('Business'),
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.school),
            title: Text('School'),
          ),
        ],
        currentIndex: _selectedIndex,
        selectedItemColor: Colors.amber[800],
        onTap: _onItemTapped,
      ),
    );
  }
}
```