Here is a link for the Society of Corporate Secretaries & Governance Professionals publication Governance for Nonprofits.  http://www.governanceprofessionals.org/nfp/nonprofits-note-readers.shtml.  The publication doesn't specifically cover the audit committee function, but it is a good read anyway.  For a detailed nonprofit audit committee discussion paper, click on the D&O and audit committee link on my website, http://davidtate.us.