This is my first time doing an internal audit on "sales and marketing". I feel like our company doesn't have a sales and marketing department. How should I go about doing an internal audit for a department that doesn't get used?
I don’t know if I understand your question correctly.
ISO 9001:2015 doesn’t mention departments and has no requirements regarding marketing. So, why are you auditing sales and marketing, if you don’t have that working in your organization? ISO 9001:2015 mentions processes, not departments. Why don’t you audit processes? About sales, use ISO 9001:2015 clause 8.2.
Another possibility is that your organization has not yet implemented a quality management system. If that is the case, consider doing a gap analysis: