Musical seats?
I am reluctant to suggest this given the emphasis on sliding surfaces, bases or pivot points, but here goes.
If your seat(s) is/are original horse-hair-padded affairs, there may be nothing you can do short of disassembling them to find the source of the music. As the padding decomposes (loses its hair) it leaves behind a wire framework that has no insulation, thus the noise. This can happen with most seats that have internal springs not unlike that old sofa with the protruding springs! I recall trying to find a similar problem. While everyone thought the sound was in the car, the squeak turned out to be a loose muffler bracket.
My point is that not all noises are what they seem.