Book 9 – Having in the preceding book shown that the worship of demons must be abjured, since they in a thousand ways proclaim themselves to be wicked spirits, Augustine in this book meets those who allege a distinction among demons, some being evil, while others are good; and, having exploded this distinction, he proves that to no demon, but to Christ alone, belongs the office of providing men with eternal blessedness.
Please refer to this link for the audio. Sorry for any inconvenience!