From an ISO9001 point of view how should the audit be approached?
As you would any other area of the system - focus on the
processes. There are various 'models' for project management office - anything from virtually running/coordinating one or multiple projects through to more or less functionig as a support office to individual project managers. So it really depends on what the PMO does in
your organisation.
But as Andy says also, think processes. Stop thinking 'functional department' (as in, I'm going to audit the PMO or the HR Department etc) and instead focus on your
processes.
So, for example, if you're following a particular process ('new product/ service', say, or 'change management process') follow it though its various phases and see what part each role/functional area plays and how well they interact. Then you can also look at what their responsibilities & authorities are, whether there are the requirements for competency including records, if there are any documented procedures, if they are being followed, etc. Particularly look at the interfaces between functions - wherever information is passed between different departments is often a place of potential problems (missed information, dropped balls etc).
And yes, ISO 10006 Guidelines for Quality Management in Projects is useful guidance on what to look for - but not just in the PMO, across the range of project processes.