The mixer controls the audio and comes with Pygame, so it is not a script I created. What operation system are you using? I found another user with the same issue on Stackoverflow, who apparently uses some Linux OS.
Did you change anything in the code? According to this Reddit thread, Pygame has to be initialised before it can use mixer. It is initialised in MAIN.py
To test whether the mixer is correctly installed, try opening IDLE console and type the following:
(from pygame import mixer (ENTER) mixer.init() (ENTER) and you should see this:
>>> from pygame import mixer pygame 2.0.1 (SDL 2.0.14, Python 3.7.4) Hello from the pygame community. <a href="https://www.pygame.org/contribute.html >>>">https://www.pygame.org/contribute.html >>></a> mixer.init() >>>
If this causes an error, you may have had an error in the Pygame installation. I hope it can be used :-)