A few of our parents have their own businesses and one is a CPA. Our audit committee is 3 people who meet with the treasurer only and go over the books. Never have we allowed any officer or committee chairperson to be on this committee.
Of course, no check signer should be on the audit committee. Kinda defeats the purpose of an "independent review". Naturally the treasurer needs to be available to the audit committee to answer questions, but should not be driving the process.
For our PTO, we ask for a volunteer from the parent community to review our books each summer (our fiscal year ends July 31). We have a checklist/worksheet we provide to help the volunteer, but none of the PTO officers work on the review. When the reviewer is finished, she gives the board an assessment of the status of the books and feedback on our financial controls. We don't do a formal, legal audit, so the format of the review changes a little from year to year depending on the experience of our volunteer, but the basic approach has worked well for us.