Box it. There are two ways to go about this. The first option is to build a box that will just barely fit the fan, bolt or screw in fan to bottom plate, cut the holes needed on each side and connect whatever ducting you will be using, seal it up around where the ducting and box meet so it's best to cut holes the exact size of your ducting or larger if it is insulated ducting which is best for soundproofing, wrap the fan in saran wrap then use a crack fill aerosol can(not a times x4 expanding but the crack fill kind x2-3 expansion) and fill it up as best you can so the fan is completely or almost completely covered, let dry and seal the top plate on. Done. You just have to remember to drill a hole for the power and speed control cords. Hang it from something in the attic with bungee cords or the racket ropes sold to hang carbon filters.
Second option is the same thing except you build a big box(anywhere from 4-10 times the size of your fan) and connect your ducting and seal the holes you cut the same way, but instead of using filler spray you just pack old clothing and insulation in there.
I've seen both types and have done the small box option before for a friend and when using insulated ducting it quiets the fan by more than 50%.
Cheaper and quieter than any duct muffler. Must hang it though to deaden vibrations which is what sound basically is.