你好，我按照老师的做了，为什么播放不了

pygame,sys

import constants


def main():
   pass
   # intialize
   pygame.init()

   # size of the screen
   # depend on the resolution of the drawing
   width, height = 480, 852

   # make screen
   screen = pygame.display.set_mode((width, height))
   # name the window
   pygame.display.set_caption("Jet Fighter")

   # load backgroud pictrue
   bg = pygame.image.load(constants.BG_PIC)

   # load BGM
   pygame.mixer.music.load(constants.BG_MUSIC)
   # repeat BGM
   pygame.mixer.music.play(-1)
   # set volume
   pygame.mixer.music.set_volume(0.2)

   while True:
       for event in pygame.event.get():
           # exist the game
           if event.type == pygame.QUIT:
               pygame.quit()
               sys.exit()

       #draw the background pic
       screen.blit(bg,bg.get_rect())
       pygame.display.flip()


if __name__ == "__main__":
   main()

import os

# root of the project
BASE_DIR = os.path.dirname(os.path.abspath(__file__))
# static files directory
STATIC_DIR = os.path.join(BASE_DIR, "static_resources")

# Backgound pic
BG_PIC = os.path.join(STATIC_DIR, "images/background.png")

# BGM
BG_MUSIC = os.path.join(STATIC_DIR, "bgm/game_bg_music.mp3")