请教，QT中这种未定义报错，怎么办

void Widget::bgMusic()
{
    QMediaPlaylist *playlist = new QMediaPlaylist;
    playlist->addMedia(QUrl("qrc:/02-Hans Zimmer-Cornfield Chase.wav"));//资源路径
    playlist->setCurrentIndex(1);
    playlist->setPlaybackMode(QMediaPlaylist::CurrentItemInLoop);//循环模式

    QMediaPlayer *player = new QMediaPlayer;
    player->setPlaylist(playlist);
    player->setVolume(voice);//音量大小

    player->play();
}

void Widget::on_VoiceSlider_valueChanged(int value)
{
    //player->setVolume(value);//音量大小
}

把QMediaPlayer *player = new QMediaPlayer;语句放到函数的外面定义即可；也就是定义全局变量。