If such an audit happened and it turned up widespread abuses, do you think it would ever be released? Which is why such an audit hasn't been ordered and probably never will be; at least not publicly. Even if this is going on, it unfortunately probably needs to be cleaned up quietly (i.e. some judges resign, some executives of prison companies sent to jail over accounting irregularities, etc.)
I'm willing to bet this is more widespread than we imagine. It's pretty well documented that the justice system targets young black males who receive longer sentences and are more likely to be sent to prison for minor offenses than white offenders of a similar background. And why not? They're less likely to complain, and less likely to be taken seriously when they do because of the stereotype that black men go to prison. As a result, many in poor black communities trust the gang members selling drugs on the corner more than the police (see the whole anti-snitching movement).
I know this thread wasn't originally about race, but it's impossible to talk about prisons without talking about race because the justice system is so inherently racist. Just ask yourself if this judge would have gotten caught if he had only sent young black kids to jail. Then ask yourself how many white, well-educated judges are smart enough to realize that. I'm sure the vast majority of judges take their job seriously and would be appalled at this kind of behavior, but it doesn't take many to seriously undermine confidence in the system.