well, you could shave a channel in the sides of the slider a la dampened Alps and quiet it that way... but would probably be cheaper/easier to mold some custom MX sliders if one were to do that (and would likely be expensive for a small scale DIY mod project.)
That said, I am typing this on a Filco w/ straight up clears (yes, I know that Filco never made a board like that ) with Teflon-lubed sliders and Cherry corp. doubleshots - it's not *quite* as silent as a good RD board typed on by someone who has a light touch, but it is still a very quiet board by any standards let alone mechanical keyboard standards. I thought about putting some Dynamat on the plate in areas that don't show with the cover on, but a) I don't have any pinging problem with this board, either before or after the mods, and I suspect that that is the main thing that it would reduce and b) I didn't have any dynamat handy when I had the board apart...
I also lubed the heck out of the stabilizers and used a grey switch for the space bar so that I'm not bottoming out on anything. That helps quite a bit.
To put the sound in perspective, when I'm sitting at my desk at work and typing on the Filco, it sounds about as loud to me as the sound I can hear when the person in the next cube over is typing on her new Dell rubber dome board.
And yes, the sound of the switches topping out is the loudest sound that you can hear when I'm typing.