Soundproof basement and bedroom vents
We line the ducts with “ductboard” insulation and use sound baffles. Often we’re concerned about hearing the  sound of the fan coming on or turning off. We’ll use sound baffles to block the clicking sound from traveling down the duct. However, we usually “up size” the ducts to accommodate the added “blockage” in the duct. 

Also, we design the system so no two vent/grille openings are too close together without sound having to travel about 10’ - 12’ before coming out the next vent.