I am being audited for American Opportunity Tax Credit

Yep, there's a lot of that, especially since it's a refundable credit.

First, try to understand what and why they are questioning it.  A lot of these IRS leteers appear to be coming form nothing more than the school left box 1 of the 1098-T blank. The most frequently, they only want you to prove that you actually paid $4000+ of qualified expenses.That's just a matter of providing copies of school billing statements and cancelled checks or checking account or credit card statements

But, a lot of people are just not eligible. See https://www.irs.gov/Individuals/AOTC

There's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A student, under age 24, is only eligible for the refundable portion of the American Opportunity Credit if he supports himself by working . You cannot be supporting yourself on parental support, 529 plans or student loans & grants. You must have actually paid tuition, not had it paid by scholarships & grants.  It is usually best if the parent claims that credit.  

https://support.turbotax.intuit.com/irs-notice/audit-support/

See  I received a letter from the IRS questioning my AOTC claim. What should I do?

https://www.irs.gov/individuals/education-credits-questions-and-answers