I have seen what is claimed to be a "noisless" soil pipe system at a
trade show.

It is made by Wavin plastics, but I couldn't find any mention of it on
their web site.

My question is this: How can a single-wall pipe be noiseless? the
water flows in it just like others, I suppose...

I am thinking to replace the pipes (from upstairs toilet) in my
downstairs toilet with the noisless ones, but am wondering if this is
just a sellers claim.

They say that they are good for hotels etc.