The Matias switches are quieter than most other mechanical switches but the switches are only one part of how a keyboard can be loud. All metal springs vibrate, and I think that most of the sound is caused by residual vibration that has propagated through the plate and case.
There are quite a few old threads already about how to silence keyboards. One of the easiest methods is often to place the keyboard on a large mousepad. There are mousepads that cover a large portion of the desk.
Some people have put sound-dampening material inside their keyboards. I would try any of those ideas before trying to modify the switches.
Matias/Alps switches can be opened from the top with tweezers without the need to remove them from the PCB. I guess that lubricating moving parts inside could help a tiny bit... but only a tiny bit. As I said, the Matias Quiet switch is already one of the quietest mechanical switches there are. Lubrication is time-consuming, fiddly and requires a lube that is friendly to both metal and plastics.
