It looks like you are providing a stereo file but for some reason the sound device will only support mono. Try converting a copy of one of your files from stereo to mono (assuming that doesn’t break some requirement you have), using something like Audacity.
Was that the full error message text?
EDIT: Actually, first you might just want to try forcing the sound to be stereo, rather than allowing it to guess, as follows:
slot_machine_ding_sound = sound.Sound(os.path.join(slot_machine_path, ‘Sound02 Ding.wav’),
stereo = True)