Since no one has refuted that claim
Actually, I have heard it mentioned MANY times that if Trump were under audit he could have provided the letter from the IRS informing him that he is being audited. He has never provided proof that he is being or has been audited.
So, it is NOT certain that he's been audited for any tax year prior to 2016. It IS certain that, by law, he will be audited EVERY year he is president. :-)