So, depending on how close your speakers are, room construction, type of AC outlet and overall system performance it could make a nice improvement in sound when you swap out the cover.
A 2 3/4" by 4 1/2" electrical cover of surface area 12 square inches in an average 16 by 12 by 8 ft room with total surface area of roughly 120,000 square inches.
This makes less than 1 hundredth of 1 percent of the surface area???
Surely, in order to make a "nice improvement in sound" then
1) the cover would need to be totally loose and rattling terribly - unusual but possible.
2) An extremely stiff power lead connected to a humming device (transformer or shaver or something like that) - again unusual
3) next to one's ears (listening position on the floor near a wall) - again unusual
I can't see that being simply close to a speaker would be enough. I also can't see how changing the cover material could be possibly be enough to be audible under ordinary circumstances.
Have I experimented like Freemand? No....but come on guys! It ain't April the 1st is it?