The annual reports are signed off by Deloitte Touche See page 327 of the PDF version of the 2014 report. Also see the section titled Federal Reserve System Audits listed in the table of contents.
If the Fed is fully audited, why does Janet Yellen oppose it? Yeah, think about it.
“I strongly oppose Audit the Fed,” said Federal Reserve chief Janet Yellen, saying that it would “bring short-term political pressures to bear” on the central bank and dissuade it from making the “hard choices” needed to curb inflation.
http://fortune.com/2015/02/24/federal-reserve-janet-yellen-audit/