I've had my Model M for about two weeks now and love it, but the damn thing is terribly loud and it makes me a little bit uncomfortable as to others suffering it. So I bought some floss and modded it! As I haven't been able to find recorded samples of this mod before, I'm uploading some for future reference so anyone can use it.

Ultra is a tad bit thinner, but looks more dense, or at least it's woven tighter. The good thing is it's easier to cut and maintains the same thickness (I don't really think it matters but yeah). It also smells like mint. Both boxes are enough for one keyboard, as you only need about 1.5cm of floss per key. The ultra packs 25m, and the super 4m total of spongy material. They cost me the same, about 5€ each in Spain. They both go in and out easily.

Left: ultra | Right: super
So now to the important part: get the mp3 https://mega.co.nz/#!MYdkkQKJ!IWK-I2WjA ... yImhGfggWc
Disclaimer: it's recorded with an iPhone4, I think it sounds pretty good but do your own mental adjusting if you know how a Model M sounds. If not, well... I've added the normal key for reference, and I'd say overall in real life it sounds a little sharper and higher pitch. Use headphones!
The first thing you'll hear is a normal key, then the ultra flossed, and finally the super.
Ultra: It's at least half as noisy, even two thirds less. The pinging echo is gone, instead it sound a bit like the crackling noise of two wires touching, a gentle crick. I think it's also a bit harder, but it maintains the feel of the key. If you only want to damp the sound I'd say this is the one. I tried doing two strings together but the key gets pretty hard to press and didn't like the feel at all, also it didn't look worth the hassle comparing it to the super (below) if you wanted the quieter version.
Super: Even quieter, it is noticeable even comparing it to the ultra, it even dampens more the click. At this point what makes all the noise is the plastic rubbing against each other. The feel is pretty much gone, it sounds more in a low pitch. To me (but I'm not a connoiseur) it sounds a bit like ALPS or cherry browns, but with just a tiny click in the background and a bit noisier. I'd say it's as hard to press as the ultra, so just a bit more than a regular one. The feel is pretty good anyway, although different, at least to me.
I'm not sure which I'll do, I've only tried some keys, but I'm leaning towards the super, just because I'm in a situation where I don't want to disturb people. To be honest, I still think the natural sound of the Model M is the best one, but if you, like me, want a quieter one, both of these are really good too.
EDIT: INFO ON THE PROCESS AND CONCLUSION
Some days have passed and I've flossed the keyboard with the super as I was thinking previously. It took some time and tries to figure out the way to do it too. What you want to do is insert the string of floss into the spring and leave at least 4mm outside. When placing the key try to make it so the string bends towards the front, you can go as far as cutting a little edge on that extra floss to help it. I kept thinking the other day about the sound it makes and couldn't come up with what it reminded me of, but I finally got it, it sounds like arcade buttons! It is possible to make them completely silent, which seems really weird, because it also lacks the feel of the actuation point, but I've no idea how to do that, it's happened in three or four keys but I can't replicate it on purpose.
I'm very happy, enjoying the sound and the feel of the keys. I've used several other people to test the differences fearing that I was getting paranoid about sounds and textures and they more or less agree on what I wrote, so to recap: lower pitch, damped sounds, no spring echo, some clicky feel, super sucks more noise than ultra.
It's not as cool as the normal sound and feel, but I think we all know that already

PS: If you find the post and the recording worthy, feel free to share it wherever it may be useful. No need to credit or anything.