Are you trying to deaden the sound between the floors or the sound that travels through the duct from lets say the upstairs bedroom duct to the downstairs bedroom duct? I don't know of any particular ways to keep you and your renter from eavesdropping on each others bedrooms except to keep the duct work as far as apart as possible and buy quieter box springs. You could run the furnace fan during those private times.
Between the floors the cheapest is fiberglass batt insulation but type control and f and do a search for sound as we have talked about it before and listed some websites that deal with it. I not so sure that fiberglass round ducts is your answer to either question. It is the air inside that carries sound and the pipe acts like a funnel. Try talking into a long tube such as like what holds wrapping paper to see how it funnels your voice.