as is famously discussed, al capone went down for tax evasion, not for the crimes he committed to make his money. you can't get caught from an irs audit. it's what's referred to as a "silver platter" situation, the irs can't hand you over when all they were doing was audited. this is the same way that evidence found in an illegal search can't be used in court, you can announce that you are somebody's drug dealer at at their murder trial and not be busted, etc. if the irs could turn you in on an audit, you could probably invoke the 5th amendment to skip whatever illegal aspects there were to avoid incriminating yourself.
just found a particularly amusing example:
http://www.ksrevenue.org/perstaxtypesdrug.htm