I'm taking azithromycin for sinus infection..can i drink a cocktail with dinner?

You shouldn't: You should not drink alcohol while sick because it suppresses the immune system and you are not allowing your body to fight the infection. On a separate note not sure whether the alcohol can influence the metabolism of the antibiotic or even have a reaction with the antibiotic. Either way not a good idea.

No: You should not take any alcoholic beverages while using any medication and not with zithromax