求高手指教，关于DoubleVar（），程序已有，但是不知道为什么不合适。

from tkinter import *
import pygame.mixer

app=Tk()
app.title("Head First Mix")

sound_file=("D:\\PythonX\\Lib\\site-packages\\pygame\\examples\\data\\boom.wav")

mixer=pygame.mixer
mixer.init()
            
def track_toggles():
    if track_playing.get()==1:
        track.play(loops=-1)
    else:
        track.stop()
        
def change_volume(v):
    track.set_volume(volume.get())
   
track=mixer.Sound(sound_file)
track_playing=IntVar()
track_button=Checkbutton(app,variable=track_playing,command=track_toggles,text="Play/Stop").pack(side=LEFT)

#volume=DoubleVar()
volume.set(track.get_volume())
volume_scale=Scale(variable=volume,from_=0.0,to=1.0,resolution=0.1,command=change_volume,label="Volume",orient=HORIZONTAL)
volume_scale.pack(side=RIGHT)            

def shutdown():
    track.stop()
    app.destroy()
   
app.protocol("WM_DELETE_WINDOW",shutdown)

app.mainloop()