Why do anything? IRS auditors are instructed to close audits within 28 months of the date you filed your tax return or the date it was due (April 15), whichever is later. For example, if you filed your 2009 return on April 15, 2010, the IRS wants the audit completed by August 15, 2012. Legally, the IRS has an additional eight months (until April 14, 2013), but auditors are instructed to complete the audit with at least eight months to spare so the IRS has time to process any appeals.
If you haven't heard from the auditor, it could mean a number of things. The auditor may have been transferred or terminated. Or your file may be sitting in a pile awaiting processing somewhere in the IRS. When your file resurfaces, a new auditor is under a deadline to close it, which can work in your favor. And, in the best of all worlds, the time limit for completing the audit may expire while your file is in IRS never-never land. So leave the sleeping dog alone.
For more tips on handling an audit, see Nolo's Checklist: How to Survive a Tax Audit.