The speed of electricity is conceptually the speed of the electromagnetic signal in the wire which is somewhat similar to the concept of the speed of light in a transparent medium. In fact mass is the most concentrated form of energy known. Equivalently the mass of a particle at rest is equal to its energy E divided by the speed of light squared c2.
The formula defines the energy E of a particle in its rest frame as the product of mass m with the speed of light squared c2.
The speed of light in a vacuum is 983 571 05643 feet per second 299 792 458 meters per second. The index of refraction is proportional to the speed of light in a material. While the speed of sound is a property of material density and temperature the maximum speed of light is intrinsic to the way matter and energy interact. The phase velocity of a guided wave in a wave guide is greater than the speed of light because the wavelength of the guided wave is basically greater then the corresponding wave length in free space.