On my car, i used to have all four speakers working correctly on a Sony Xplod setup until randomly 2 of them have decided to not work anymore. I checked with a meter that they're recieving voltage and current, but they arent making any sound.

And the even stranger part of it is that its the right front and the left-rear that arent working, and if you turn the Fade to full right, neither of the speakers work even though they're on opposite sides.