I found some very heavy cast brass covers at a second hand store years ago. they are all 3/16" to 1/4" thick in a lovely design. I use them in the living room with my stereo, but I NEVER put them in there to alter the sound. Lucky for me they are very nice loking, and what they do to the sound is obviously not actually hearable.
Elizabeth, at the time when you put the new heavy brass plates on your receptacles you would not have necessarily noticed a change in the sound from your system. You were not even thinking of the plates having anything to do with the sound from your system. Your were only concerned with the appearance of the covers. With just a little bit of your time you could check if the plates are indeed causing an effect on the sound from your system. Take just a few minutes and pull the brass plates and replace them with the regular plates you have in your home. Please post back with your findings.
Jim