I just had a review audit which showed a highly voted answer to the original question but showed it with 0 votes. I thought some user just copied the original answer and was spamming the system so I flagged it. And apparently I've failed the audit. What's the purpose of this ridiculous audit which was obviously misleading?
Unless I am not understanding what you mean by "misleading", there is absolutely nothing misleading about the audit. You simply over-thought it.
You saw a post in review, clicked through to see the actual question like a good reviewer should do and found the answer you were asked to review.
That's when you should have realized it was an audit, went back to the review and either upvoted or clicked "Looks Good" and moved on to the next post.
However, you may not have realized that there are audit posts in the review queues and, as such, assumed it was someone copying and pasting an answer given the inconsistency in the post's score. So you decided to flag the answer in the review. Nothing you did here was inherently wrong or incorrect, except you just missed the part when you should have recognized the audit post.
The only "failure" in your logic was that if it was someone trying to score free rep and copying & pasting the answer, you would have seen both the upvoted original and the 0-score duplicate answer under the question. It is a misunderstanding that I've seen other new reviewers make, but posts in the review queues are posted before reviewing, not after, so you are reviewing questions & answers as they appear live on the site (except for audits, which are tweaked slightly to disguise the fact that they are audits)
As for audits themselves, they are supposed to be obvious.
They are not designed to be "teaching tools" but to make it easy for someone who is paying attention and trap someone who robotically clicks "Looks Good" when reviewing. Everyone gets the audits, even moderators, and everyone fails audits because the audit question selection process is not perfect.
It is not until you fail multiple audits will anything happen.