Sound proofing HVAC ducts
You could try having the ducts Aerosol Duct Sealed. An expensive process not intended for it's acoustical value, but I assume it would offer some. 

Reduce noise from forced air furnace. -More related to unit noise then from people, but you still have two basic options: baffles or insulation.

Other options: move the TV so it doesn't shoot at the registers; add a white noise generator; or get new roommates\children.