不支持iPhone设置

我的iPhone应用程序有以下问题:刚安装,当我用以下代码读出我的“播放声音”偏好时:

defaults = [NSUserDefaults standardUserDefaults];
NSLog(@"Play Sounds? %d", [defaults boolForKey:@"play_sounds_preference"]);

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>StringsTable</key>
    <string>Root</string>
    <key>PreferenceSpecifiers</key>
    <array>
        <dict>
            <key>Type</key>
            <string>PSGroupSpecifier</string>
            <key>Title</key>
                <string>General Settings</string>
            </dict>
            <dict>
                <key>Type</key>
                    <string>PSToggleSwitchSpecifier</string>
                    <key>Title</key>
                    <string>Sounds</string>
                    <key>Key</key>
                    <string>play_sounds_preference</string>
                    <key>DefaultValue</key>
                    <true/>
            </dict>
    </array>
</dict>
</plist>

当用户打开Settings.app并导航到我的应用程序名称时,即使用户没有更改任何内容,设置也会显示为true。