I was on site today with the electrician, marking the walls for speaker wiring locations, etc. As luck would have it, the builder's architect showed up coincidentally at the same time.
I found out that there will actually be 3 trunk lines going through the ceiling of the media room (a trunk for the upstairs; a trunk for the main floor and a return). However, there will be no soffits or duct chases in the media room. The drop ceiling will cover all the trunks.
So the room will be a perfect rectangle as on the diagram (with the drop ceiling). So I will get the space expected, without subcontractors messing it up. The only blip being the known post in the room.
Fortunately, the electrician was there getting this information too, who will now communicate with the HVAC sub, to make sure the recessed lighting in the media room can go where we want between the trunk lines. As we marked out those locations today too.
The architect indicated that the ducts should be quiet and will not vibrate from a subwoofer or when the furnace/AC goes on/off. The trunks will all be insulated.
I hope he's right.