The Code says you are to, “have a STC rating of not less than 50 (45 If field tested) for air borne noise” in walls between dwelling units. (See ICC Section 1207.2)
Also, any “penetrations for piping, cabinets, bathtubs, heating, plumbing, etc. shall be sealed, lined, insulated, or otherwise treated to maintain the required ratings.” (Section 1207.2)
Also, “Floor/ ceiling assemblies between dwelling units or between dwelling units. And service areas shall have an impact insulation class (IIC) rating of not less than 50 (45 If field tested).” (Section 1207.3)
A standard wall (2x4 framing with gypsum board on each side) will not meet this requirement.
I’d start with removing a few electrical cover plates and see if the electrical box is sealed. I suspect this will give you a good indication of the type of construction in the party wall.