There's a thin rubber layer (that you can see in the photo), so this helps absorb some of the sound. (solesociety.com)
Abrasion resistant inner / outer rubber layer with inside aramid braiding, Working Pressure: 10 bar, Sold by the meter. (pelicanparts.com)
Then, less than a year in, I noticed the rear wheels wearing thin along the centre, and in no time the superficial rubber layer of the wheels SPLIT IN HALF! (momsstrollerreviews.com)