Adding sound effects in Kivy is very simple. A Boom instance will produce a sound when it is created, and this will happen every time a shot or missile is fired. Here is the code for boom.py:

52. # File name: boom.py
53. from kivy.uix.image import Image
54. from kivy.core.audio import SoundLoader
55. 
56. class Boom(Image):
57.   sound = SoundLoader.load('boom.wav')
58.   def boom(self, **kwargs):
59.     self.__class__.sound.play()
60.     super(Boom, self).__init__(**kwargs)

Reproducing a sound involves the use of two classes, Sound and SoundLoader (line 54). SoundLoader loads an audio file (.wav) and returns a Sound instance (line 57) that we keep in the sound reference (a static attribute of the Boom class). We play a sound every time a new Boom instance is created.