The only difference between what is called leaded glass and that which isn't is the amount of lead used in the formulation. You actually ALMOST answered your own comment in the statement about the definition of Dielectric etc. I didn't claim that the lead makes a conductive path but that I DIDN"t know if it MIGHT have an effect on its properties. i.e Conductivity, Capacitance, Inductance...
I do not think that leaded glass if conductive.
If it were to sound different then I think that the inductance or capacitance would change, but I think that the only way to affect capacitance in an insulator of a fixed geometry, is with its dielectric constant.
One of the jobs I worked back in the day required me to get certified as a Master Mechanic. That was right before I went to USC to study Electronic Engineering.
So you spent some time in SoCal.