Seriously, what is this evil effect that carpet has on signals in cables? And what is the audible effect?
Static electricity and the carpet acting as a dielectric.
We've all seen the potential for static charges to build up on a carpet. You end up with a DC charge of variable strength along the length of the cable lying in the carpet.
Carpet also acts as a dielectric. As the signal propagates down the cable, the radial component interacts with the dielectric covering over the wire. Carpeting adds to, or extends the dielectric, changing the designed behavior of the cable.
Whether or not this affects a specific cable, carpet, system, and listener combination isn't going to be knowable without listening. And many will cling to the pseudoscience that it doesn't matter and it's all in the listener's head.