1999 Pontiac Grand Factory Installed Stereo Sound Problems

1999 Pontiac Grand AM GT

When I listen to the radio or to a CD, the sound is distorted. It actually sounds as if someone is turning the volumn knob up and down. I have tried listening to each speaker at a time and I hear that in every one.

What could be causing this?
Would installing an after market CD player help? You have posted twice on this car and both are related to electrical problems.
My opinion is that the ignition switch needs replacing. This is the part that is connected to the key switch. It turns the electric on. Are other accessories in the car seem to have that fading effect or is it just the stereo?