May as well retake the course. As mentioned above, many (most?) schools don’t give credit for AP courses for pre-reqs. And if all you need is a refresher, then you could probably get a decent grade and that’s boost your sGPA.
I don’t see any reason to audit in this case. You’ll take the class more seriously and learn more if you have to get a grade.
I was surprised at how much more I learned in my college courses than the same course (whether normal, honors, or AP) in HS.