Carpet and padding. Almost everything else would have had to be designed into the construction. A hard surface home is always NOISY. Adding soft surfaces is the only thing that fixes that.

Cork 6mm or 12mm thickness will get you the "most for the least". Which is to say, The most noise reduction for the least amount of money.

Stuffing the joists with Roxul or Safe N Sound (two layers for best results) and then mounting acoustic drywall ($3.50/sf) over the Safe N Sound will triple what you have right now. Something like that should be $5-$7/sf.

In essence, you have built an echo chamber. Now you have to counter act all those beautiful aspects with noise reducing materials. Plan for it now....a retrofit for acoustic insulation is one of the most expensive retrofits you will EVER attempt (it normally tops out at $10/sf with a retrofit).

Good luck. If you have BIG problems with noise, go with 12mm Cork underlay underneath your hard surface flooring = 22 dB reduction. Compare that to acoustic drywall = 18 dB reduction. And the cork is HALF the price of the drywall - and easier/cheaper to install.

Carpet reduces echo....but will do very little to air borne noises (like a cough) moving "up-down" from one level to another. The carpet does not have an acoustic rating....but the pad does. A pad with a high "rubber" content (real rubber - like the black round things attached to your car) will be the most effective against airborne noises. Carpet removes "foot steps" but that's ALL it is good for. It will do nothing against TV noise, conversations, etc.