Where would I find audit requirements for these types of mortgages that are only serviced and then sold through the bank?

Are you the orignator, the seller, the purchaser, the servicer? To which secondary market are you referring? In order to receive any meaningful responses, you are going to have to be a little more specific.

One of the best places to start is the seller/servicer agreement your bank will execute with the investor (assuming you are the seller). Since you are originating to sell into the secondary market, I also assume you have an independent Quality Control department. The QC procedures also would be a place to start for file review. Hope this helps.