如何在 PyQt5 中进行音量缩放，并在更改时将其音量更改为用户指定的音量

对不起，你的代码很重，里面包含了很多不必要的东西。
您还可以使用晦涩的函数来填充
小部件对象（def Button()等def Button_2()）。

您使用小部件的绝对定位，例如：

        btn_name.resize(resize, resize_2)
        btn_name.move(move, move_2)
        

因此，在我的屏幕上，除了“开始”和“音量设置”按钮名称的文本外，我看不到您的应用程序窗口的完整图片。
您需要学习布局管理部分 并在应用程序中使用布局管理器。

您正在滥用：

self.widget.show()
self.widget.close()

这些都是非常昂贵的操作，以这种方式使用它们是不正确的。

现在进入问题的重点。

我创建了一个滑块对象来改变音量：

self.slider = QSlider(Qt.Horizontal, self)
...

单击“音量设置”按钮时显示，再次单击同一按钮时隐藏。
注意，单击“开始”按钮后，滑块与某种重叠，label无法与之交互。因此，您需要以某种方式以不同的方式放置它，或者，正如我上面写的，在Layout Management中使用小部件的放置。

import sys
from PyQt5 import QtWidgets, QtGui, QtCore, QtMultimedia
from PyQt5.Qt import *

# ??? зачем вам это ???
# https://doc.qt.io/qt-5/qtwin-obsolete.html#setCurrentProcessExplicitAppUserModelID
# Эта функция устарела. Он предназначен для сохранения работоспособности 
# старого исходного кода. Мы настоятельно не рекомендуем использовать его в новом коде.
# ???try:
# ???    from PyQt5.QtWinExtras import QtWin
# ???    myappid = 'mycompany.myproduct.subproduct.version'
# ???    QtWin.setCurrentProcessExplicitAppUserModelID(myappid)
# ???except ImportError:
# ???    pass
    
# ??? зачем вам это ???
# вы это негде не используете    
# ???class Button(QPushButton):
# ???    mouseMoved = pyqtSignal()
# ???    def mouseMoveEvent(self, event):
# ???        self.mouseMoved.emit()


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
#       filename = 'music.mp3'                          # установите свое music.mp3       
        filename = 'D:/_Qt/Mp3/zvuki_prirody-td.wav'
    
        fullpath = QtCore.QDir.current().absoluteFilePath(filename) 
        url = QtCore.QUrl.fromLocalFile(fullpath)
        content = QtMultimedia.QMediaContent(url)
        self.player = QtMultimedia.QMediaPlayer()
        self.player.setMedia(content)
        self.player.setVolume(50.0)
        self.player.play()        
     
        self.slider = QSlider(Qt.Horizontal, self)           
        self.slider.setRange(0, 100)
        self.slider.setValue(50)
        self.slider.setGeometry(800, 620, 300, 70)
        self.slider.hide()
        self.slider.valueChanged.connect(self.player.setVolume)
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
        
        def Button(btn_name, text, color1, px, resize, resize_2, move, move_2, color):
            btn_name.setMouseTracking(True);
            btn_name.setText(text)
            btn_name.setStyleSheet(
                "background-color: " + color1 + "; color: " + color + "; border-radius: " + str(px) + "px")
            btn_name.setFont(QtGui.QFont('MS Serif', 14))
            btn_name.resize(resize, resize_2)
            btn_name.move(move, move_2)
            #btn_name.clicked.connect(mains)
            
        def Button_2(btn_name, text, px, resize, resize_2, move, move_2, color, size):
            btn_name.setMouseTracking(True);
            btn_name.setText(text)
            btn_name.setStyleSheet(
                "color: " + color + "; border-radius: " + str(px) + "px")
            btn_name.setFont(QtGui.QFont('Gabriola', size))
            btn_name.resize(resize, resize_2)
            btn_name.move(move, move_2)
            #btn_name.clicked.connect(mains)

# !!!   self.btn_select, "Настройки громкости", 20, 300, 100, 800, 520, "Black", self.volume_custom
        def Button_3(btn_name, text, px, resize, resize_2, move, move_2, color, mains):
            btn_name.setMouseTracking(True);
# !!! 
# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
            if text == "Настройки громкости":
                btn_name.setCheckable(True)   
                btn_name.setText(f'{text}: ({self.slider.value()})')
            else:
                btn_name.setText(text)
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  

            btn_name.setStyleSheet(
                "color: " + color + "; border-radius: " + str(px) + "px")
            btn_name.setFont(QtGui.QFont('MS Serif', 14))
            btn_name.resize(resize, resize_2)
            btn_name.move(move, move_2)
            btn_name.clicked.connect(mains)

        def Image(lbl_name, move_1, move_2, resize_1, resize_2):
            lbl_name.setPixmap(self.pix)
            lbl_name.move(move_1, move_2)
            lbl_name.resize(resize_1, resize_2)

        self.lbl2 = QtWidgets.QLabel(self)
        
        self.pix = QtGui.QPixmap("lena.jpg")
        Image(self.lbl2, 0, 0, 1920, 1080)

        
        self.lbl1 = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.lbl1, 0, 800, 1920, 300)
        self.btn1 = QPushButton(self)
        Button_2(self.btn1, "Вы появились в неизвестном вам месте,\nгде было невероятно красиво", 10, 1000, 200, 430, 830, "White", 30)
        self.btn3 = QPushButton(self)
        Button_2(self.btn3, "", 10, 150, 100, -30, 500, "White", 14)
        self.inv = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.inv, -30, 600, 156, 96)
        self.btn31 = QPushButton(self)
        Button_3(self.btn31, "", 0, 100, 100, 0, 600, "White", self.inventory)
        self.inv_open = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.inv_open, 70, 40, 1770, 996)
        self.inv_open.close()
        self.btn4 = QPushButton(self)
        Button_3(self.btn4, "", 5, 100, 100, 1650, 110, "White", self.inventory_close)
        self.btn4.close()
        self.lbl20 = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.lbl20, 0, 0, 1920, 1080)
        self.lbl20.close()
        self.mg_open = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.mg_open, -30, 480, 156, 96)
        self.btn5 = QPushButton(self)
        Button_3(self.btn5, "", 0, 100, 100, 0, 480, "White", self.event_log)
        self.btn6 = QPushButton(self)
        Button_3(self.btn6, "", 0, 460, 800, 160, 100, "White", self.notes)
        self.btn6.close()
        self.btn7 = QPushButton(self)
        Button_3(self.btn7, "", 0, 460, 800, 700, 100, "White", self.easter)
        self.btn7.close()
        self.btn8 = QPushButton(self)
        Button_3(self.btn8, "", 0, 460, 800, 1260, 100, "White", self.tasks)
        self.btn8.close()
        self.notes_open = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.notes_open, 0, 0, 1920, 1080)
        self.notes_open.close()
        self.btn9 = QPushButton(self)
        Button_3(self.btn9, "", 5, 80, 80, 1730, 50, "White", self.inventory_close)
        self.btn9.close()
        self.easter_open = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.easter_open, 0, 0, 1920, 1080)
        self.easter_open.close()
        self.btn10 = QPushButton(self)
        Button_3(self.btn10, "", 5, 80, 80, 1730, 50, "White", self.inventory_close)
        self.btn10.close()

        self.tasks_open = QtWidgets.QLabel(self)
        self.pix = QtGui.QPixmap("ball.png")
        Image(self.tasks_open, 0, 0, 1920, 1080)
        self.tasks_open.close()
        self.btn11 = QPushButton(self)
        Button_3(self.btn11, "", 5, 80, 80, 1730, 50, "White", self.inventory_close)
        self.btn11.close()

        self.lbl2.close()
        self.btn31.close()
        self.btn1.close()
        self.lbl1.close()
        self.inv.close()
        self.mg_open.close()
        self.btn5.close()
        self.btn6.close()
        
        self.btn_start = QPushButton(self)
        Button_3(self.btn_start, "Начать", 20, 300, 100, 800, 400, "Black", self.start)
# !!!
        self.btn_select = QPushButton(self)
        Button_3(self.btn_select, "Настройки громкости", 20, 300, 100, 800, 520, "Black", self.volume_custom)

    def inventory(self):
        self.btn31.close()
        self.btn1.close()
        self.lbl1.close()
        self.inv.close()
        self.inv_open.show()
        self.btn4.show()
        self.mg_open.close()
        self.btn5.close()
        self.btn6.close()
        
    def inventory_close(self):
        self.btn3.show()
        self.btn1.show()
        self.lbl1.show()
        self.inv.show()
        self.inv_open.close()
        self.btn4.close()
        self.notes_open.close()
        self.btn9.close()
        self.mg_open.show()
        self.btn5.show()
        self.btn6.close()
        self.btn7.close()
        self.btn8.close()
        self.easter_open.close()
        self.btn10.close()
        self.btn11.close()
        self.tasks_open.close()
        self.btn31.show()
        
    def animations(self):
        animation = QPropertyAnimation(self)
        animation.setTargetObject(self.btn_name)
        animation.setPropertyName(b'pos')
        animation.setStartValue(QPoint(250, -200))
        animation.setEndValue(QPoint(250, 0))
        animation.setDuration(500)
        animation.start()
        
    def event_log(self):
        self.btn31.close()
        self.btn3.close()
        self.btn1.close()
        self.lbl1.close()
        self.inv.close()
        self.btn5.close()
        self.mg_open.close()
        self.lbl20.show()
        self.btn6.show()
        self.btn7.show()
        self.btn8.show()
        
    def notes(self):
        self.lbl20.close()
        self.notes_open.show()
        self.btn9.show()
        
    def easter(self):
        self.lbl2.close()
        self.easter_open.show()
        self.btn10.show()
        
    def tasks(self):
        self.lbl2.close()
        self.tasks_open.show()
        self.btn11.show()

# !!!
    def start(self):
        self.btn3.show()
        self.btn1.show()
        self.lbl1.show()
        self.inv.show()
        self.inv_open.close()
        self.btn4.close()
        self.notes_open.close()
        self.btn9.close()
        self.mg_open.show()
        self.btn5.show()
        self.btn6.close()
        self.btn7.close()
        self.btn8.close()
        self.easter_open.close()
        self.btn10.close()
        self.btn11.close()
        self.tasks_open.close()
        self.btn31.show()
        self.lbl2.show()

# !!!
# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv        
    def volume_custom(self):
        #тут действие, но я поставил print что бы не выдавал ошибку   
        sender = self.sender()
        print(f'def volume_custom(): {sender.text()} - {sender.isChecked()}')

        if sender.isChecked():
            self.slider.show()
        else:
            self.slider.hide()
            
        sender.setText(f'Настройки громкости: ({self.slider.value()})')
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    app.setWindowIcon(QtGui.QIcon('Ok.png'))
    ex = MainWindow()
    ex.setWindowTitle('Story Telling')
    ex.setWindowIcon(QIcon('Ok.png'))
    ex.showFullScreen()
    ex.show()
      
    sys.exit(app.exec_())

这是我在我的窗口中看到的，安装了我的所有图像。现在用您自己的替换所有图像并尝试。