As a finance department manager do I have to legally enter into a non-disclosure agreement with external auditors every year before the audit work is started?
Non-Disclosure Agreement With External Auditors
Typically client information confidentiality is covered in your engagement agreement with the audit firm, and thus there would be no need to do so separately each year. If they are a major firm I would be shocked if it were not be covered in your agreement. If it is a small firm, well, you might want to check the agreement to make sure it's handled.
There is no reason to re-sign a NDA unless it is written with an expiration clause. NDA's are usually written "for-ever", however in practical terms after you leave your knowledge has a declining value to your former employer. This does not relieve you of your NDA, but puts it into a more realistic focus.