file = hfmix2.py
error is on line 3

# imports
from Tkinter import *
from sound_panel2 import *
import pygame.mixer

# create gui
app = Tk()
app.title("Head First Mix")

# create mixder
mixer = pygame.mixer
mixer.init()

# call functions
panel1 = SoundPanel(app, mixer, "50459_M_RED_Nephlimizer.wav")
panel1.pack()
panel1 = SoundPanel(app, mixer, "49119_M_RED_HardBouncer.wav")
panel1.pack()

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

file = sound_panel2.py
error is on line 19

# this is a module

# imports
from Tkinter import *
import pygame.mixer

# create class
class SoundPanel(Frame):
    def __init__(self, app, mixer, sound_file):
        Frame.__init__(self, app)
        self.track = mixer.Sound(sound_file)
        self.track_playing = IntVar()
        
        track_button = Checkbutton(self, variable = self.track_playing,
                                   command = self.track_toggle, text = sound_file)
        track_button.pack(side = LEFT)
        self.volume = DoubleVar()
        self.volume.set(self.track.get_volume())
        volume_scale = Scale(self, variable = self.volume, from_ = 0.0, to 1.0,
                             resolution = 0.1, command = self.change_volume,
                             label = "Volume", orient = HORIZONTAL)
        volume_scale.pack(side = RIGHT)
        
    def track_toggle(self):
        if self.track_playing.get() == 1:
            self.track.play(loops = -1)
        else:
            self.track.stop()
            
    def change_volume(self, v):
        self.track.set_volume(sel.volume.get())
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