Some legal scholars support the idea that a sitting President can't be charged and prosecuted for criminal behavior prior to or during his term. If so, prosecuting a President would give the Judicial branch a huge power over the Executive. A potential criminal prosecution could happen after the President leaves office, if the statutes of limitation allow, or are argued to have been stayed. This has never happened and nothing has been codified, so it's all speculation.
There is the impeachment process, where a sitting President can be held accountable. It requires the Congress, specifically the house first to begin the process to determine that the President has committed "high crimes and misdemeanors." There isn't an indication or definition of what that is, or whether that includes conduct before taking office, except a low bar was set when lying about a BJ was considered a high crime/misdemeanor, an impeachable offense. As long as the House is solidly GOP, that isn't going to happen, regardless of what Thumpher has done, or will do.
As for an on-going audit, isn't the IRS an executive branch, and Thumpher their boss? He's going to direct them to keep auditing, then lose the file just before he leaves office...