You can puts ducts in a soffit so they don't necessarily need to be inside the walls. I've lived in two apartments with ducts inside soffits. Both were still very loud.
It passes though a wall, but not necessarily a wall you'd share with a neighbor typically. For instance, in my apartment we had an in-unit air handler with soffits running long the ceiling. We could hear every step on neighbors made. Most building codes require fire separation between units, so you generally wouldn't have duct work running through walls like that as it'd be a path for fire to spread.
How old was the building? Currently I'm in a noisy building — it was built in 1890, before rock wool was invented, to say nothing of fiberglass. It definitely tracks that floors tend to be worse than walls (especially bad with Euclidean zoning that encourages low ceilings and thin platforms).
