Where would I find the rules for standard level of sound insulation required in apartments for the City of Overland Park, Kansas (or Johnson County, Kansas if that helps)? I do not believe the apartment I am currently living in meets the minimum requirements as I can here my neighbors conversations quite clearly through the shared walls and ceiling.
Building codes vary pretty widely around the country. They are different for residential and commercial as well. Ultimately, the local authorities are the ones that adopt whatever code they will use and enforce. Checking with the local building department is where I would start.
There is also the time factor. The codes change every few years so it depends on when the structure was built as to what code was being followed at that time.
I do not have access to your local codes so this is something you will have to research on your own.