I own a two-story duplex (with a basement) - the units are adjacent to each other. I've been trying to minimize the sound that travels between each unit - it feels like it's the Achilles heel of what are really lovely spaces. The middle, shared wall is 2x4 studs. There are hardwood floors throughout, with no sub-floor. There is a concrete block wall in the basement that reaches the floor joists, but does not completely block off one side from the other.

What I've done so far:

Blown loose insulation into the shared wall, upstairs and downstairs.

Built out the shared wall on one side - used 2x2 studs, filled the cavities with rigid insulation, used dense drywall panels to close the whole thing up.

Noise is still traveling through, both ambient noise (loud talking, music, etc.) and walking.

I would prefer not to cover the hardwood floors, so I'm wondering if adding sub-floor material between the joists, from below, might help. (I'd also fill at least some of that cavity with Rockwool, or something similar.)

Also wondering if I should add dense drywall to the other side of the shared wall for good measure.

Other steps I'm not considering?

Any advice would be appreciated.