Is it safe to take a pm sinus med for helping with sleep while taking a zpack?

Yes. Usually the pm component is Diphenhydramine or benadryl (diphenhydramine). This is a sedating antihistamine. If you have a sinus infection and are already on antibiotics, then it is safe to use this medication to get some rest at night.
Usually. Getting adequate rest will aid in recovery from infection. These pm medications are not likely to influence the effect of azithromycin, and helping you sleep is quite beneficial.