I have used cork in the past. A company called IBL (http://www.iblproducts.com/roadbed.htm#ZCork) sells a cork roadbed that is 2.4mm thick for z scale. The cork works well and is a very traditional roadbed (in the US at least) but I find it very loud. It's especially loud if you use track nails as they will go through the cork and into the wood you have below it transferring the noise. For my next layout I'm planning on using a layer of homasoat, or some similar compressed paper product, under the cork roadbed to absorb sound. For what it's worth, I find building on expanded poly foam insulation boards (another popular method) very loud as well.

Any mix of fine ballast material in a color that matches the area you are modeling or suites your taste works well to decorate the cork. I use diluted white glue or matte medium to fix it in place applied with a dropper. Any and every book on scenery or website on the subject has a variation on the method. It takes some time but looks great once it's all done. Make sure to keep it clear of moving parts of turnouts. You don't want to glue them down. If you are going to paint and weather your rail you should do that before you ballast as well.

Good Luck.

Chris

I'm not sure what it might be called in your neck of the woods but it is essentially a compressed paper product that is used in the building industry for acoustical dampening and for some (minor) insulation value. It's similar to what they make (at least in the US) acoustical ceiling tiles out of that are used in drop ceilings like in offices etc. The stuff is nasty to cut (produces much harmful fine dust) and not always stable in environments where the humidity and temperature fluctuate (like a garage) but it is the go to roadbed (again in the US) for modelers who hand lay track. It's quiet and holds track nails well. Best practice seems to be to seal it with several coats of paint to try and control the expansion and contraction caused by humidity changes (after all it's paper.) There's a company in California that makes a product called Homabed that provides pre cut and shaped pieces for roadbed. They do make z scale roadbed but it's pretty pricey. My plan was to buy a sheet of it at a builders supply and laminate it on top of my plywood then put cork on top of that for roadbed. That would provide all the acoustical benefits of the stuff, make the track easier to spike but minimize cutting and trying to produce bevels along the edges.

I don't recall ever having seen a compressed paper product (like homasote) in any hardware store here. A substitute might be caneite (spelling?), if you can still get it. It used to be made from compressed dried sugar cane stalks by CSR.