"we understand the walls are all very well insulation, and (per city code for multi-unit structures), there is a one-hour firewall between the two parts of the building, all the way up through the attic to the roof. "

Fire walls are not designed to stop sound transmission, just reduce the rate of fire spread.

Was any acoustica insulation placed betweenthe two walls?

Extra layers of drywall? (Even on a fire rated wall more layers helps with sound transmission fron the greater mass.)

Is the extra wall steel studs?

Was sound caulk placed under the top and bottom plates of the extra wall?

The builder likely followed plans the architect provided and you approved.

Any issue lies with the architect.

no wording about this in the construction contracts

Yet a second wall was added between the units. Something's not computing.

Your problem may be flanking sounds--under and around the wall--that were not addressed.

Ductwork? We totally forgot about the air returns when we finished our basement. The ductwork was not designed by the architect and the hvac people hooked up the new air returns to the existing ones. Now the kids use them as intercoms between floors!

There is little to nothing that can be done at this point unless you are up for some deconstruction of the wall assembly. Add in some carpet and drapes and other soft surfaces that will absorb sound.

Designing structures to reduce sound transmission (STC) and impact transmission (IIC) is best done beforehand. But it's hardly hopeless to retrofit. And you don't have to tear walls down to do it. Often added mass and/or sound damping drywall in conjunction with sealants are enough to do the job. But don't forget flanking sounds; and as scrappy points out, ductwork.

Thanks to those who replied. My husband reviewed our building plans and found that Quietrock was used as the sheetrock for the the second wall that was built between the units, which is also the 1-hour firewall. We guess that the architect thought that was sufficient soundproofing.

My husband sent the drawings to another architect in our area who specifically works on soundproofing. That person replied, in part, that the new walls were not properly designed to achieve sound reduction between living units. He said, for example,

"The code minimum for a wall between units is STC 50 and with the resonances you now most likely have less than that. Most modern condo owners will have an expectation of STC 65+ - in your case where your living room is right up against [the other unit's] bedroom, it would have to go up to STC 70 for people to go about their lives and habits without hearing each other."

He referred us to the following page (which I skimmed but didn't understand) for why the design was inadequate.

http://www.greengluecompany.com/understandingTripleLeaf.php

He suggested that we would need to tear out the existing walls, including the newly-built firewall, and re-build them to properly address the resonance problem. We are hoping for a less extreme solution and are going to meet and discuss other possible remediation. (After all, what if we didn't own both units? Surely there must be a way to reduce sound transmission while working only from one side.)

If anyone reading this is going to remodel and is interested in soundproofing, please consult with someone with specific expertise during your planning stages. It was a costly error for us not to realize that was necessary.

Add a layer of 5/8" sheet rock. Recently finished a very large office layout for a defense contractor that requires quiet rooms for foreign nationals. Outside office wall and inside ceiling 1 sheet 5/8" sheet rock, inside walls 2 layers of 5/8" sheet rock all metal studding, this is Military spec. I was skeptical at first, but I assure you these offices are extremely quiet.

Call Quietrock to speak with technical services. They told me adding another layer of drywall actually reduces the STC, making transmission worse. Air is the best.

You not only need someone specialized to design, but someone specialized and meticulous to install the products. I don't know if there are certifications but who installs is very important. Be very cautious there verifying they are trained and thorough about their work or you have wasted your design and product money. These regular guys who say sure they install soundproofing materials don't know squat.

I would try putting up green glue and another (thin) layer of drywall on both sides of the wall prior to ripping out anything.

It should have had resilient channel and green glue to begin with, but it's too late for that now without deconstruction.

Sound travels through ANY opening - make sure there are no shared ductwork, etc. What about ceiling - shared attic space? Remember sound travels everywhere, the smallest missed detail will spell disaster for sound isolation.

With a double stud wall and Quietrock between the units such a poor sound separation can only be due to openings and cracks in the wall. Reducing such sound leaks is the purpose of sound insulation but it can only handle cracks.

As mentioned above I would think about sound paths above the ceilings, at electrical boxes and duct penetrations as well as shared structure but there should not be much of that at a fire wall. Quietrock and green glue won't help much if the perimeter of the panels are not sealed.

To get to STC 80 for a party wall requires more than one foot of width according to the Quiet Rock wall sections below. Also, it should be noted that Quiet Rock boards vary from 1/2"-1" with varying sound-dampening characteristics.

There are youtube videos on this subject, including retrofit for sound issues.I suggest that you research this thoroughly. The answer could be as simple as using acoustical sealant around the edges of the walls.This is such a tough problem, I wish you good luck on this.