def sound_effect(sound_file):
    pygame.mixer.init()
    pygame.mixer.Sound(sound_file)
    pygame.mixer.Sound(sound_file).play().set_volume(0.2)

def fire_bullet(si_settings, screen, ship, bullets):
"""Fire a bullet, if limit not reached yet."""
    if len(bullets) < si_settings.bullets_allowed:
        new_bullet = Bullet(si_settings, screen, ship)
        bullets.add(new_bullet)
        sound_effect('sounds/shoot.wav')`