If you can be heard through a decoupled double layered brick wall, you can also be heard through two wood frame walls standing 12 feet apart, especially when there are windows in them.

Plus, people are obviously superstitious about shared walls.

Then try concrete instead. For European mid-rise firewalls, that‘s pretty much standard.

Reading this thread has taught me that soundproofing a shared wall isn‘t possible in America.

And let‘s not forget that the average home in 1920 was way more modest than today‘s average home.