В программе при нажатии на кнопки не проигрывается звук

from kivy.app import App
from kivy.core.audio import SoundLoader
from kivy.uix.gridlayout import GridLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.label import Label
from kivy.uix.widget import Widget
from kivy.graphics import Color, Rectangle
 
class NumberApp(App):
    def btn1_pressed(self, instance):
        sound = SoundLoader.load('1.wav')
        sound.play()
    
    def btn2_pressed(self, instance):
        sound = SoundLoader.load('2.wav')
        sound.play()
            
 
    def build(self):
 
        bl = BoxLayout(orientation = 'vertical', padding=30)
        gl = GridLayout(cols = 2, spacing=3)
 
        bl.add_widget(Button(text="OK", size_hint = (1, .0), on_press = self.btn1_pressed, halign = 'auto', valign = 'middle', font_size = 96))
        
        gl.add_widget(Button(text="123456789", background_color = [.5, .0, .0, 2], size_hint = (1, .8), on_press = self.btn2_pressed, halign = 'auto', valign = 'middle', background_normal = '', font_size = 96))
        gl.add_widget( Widget() )
 
        bl.add_widget( gl )
        return bl
 
if __name__ == "__main__":
    NumberApp().run()