Class D Technology

kijanki

...  electricity in speaker cable flows in both directions canceling the most of RF (if any) ...

There's nothing inherent about an AC circuit that makes it immune to RF, so it isn't clear what your point is here.  

When electricity flows in two wires in opposite direction it tends to cancel radiated EMF or capacitively coupled noise.  Same goes for receiving - the closer two speaker wires are - the less susceptible they are to pick up anything.  Twisted pair exposes both wires symmetrically to external fields making them almost immune to coupled or radiated noise.  That's why in any circuit design signal and return should always go closely together to avoid any loops.   The point of my post was that cable (two wires where electricity flows in opposite direction) is a very poor antenna for radiating and receiving.  Twisted pair is practically no antenna at all.

There is about 1% of switching voltage noise on the speaker cable since filter is not perfect but, as I said before, 1/300 wavelength antenna won't radiate.  In addition, electricity in speaker cable flows in both directions canceling the most of RF (if any).  When you place radio directly on the wire you might get some capacitive coupling, but I doubt it.  Many speaker cables are twisted making it completely inefective (better than shield) for electromagnetic radiation or capacitive coupling (both ways - from cable or to cable).

You may find this of interest:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3977
scroll down to:
Minimizing EMI with Spread-Spectrum Modulation

Perhaps this explanation will be easier to understand:  If you make loop of a wire conducting electric AC current the radiated EMF will be proportional to loop area.  If you reduce area to almost zero (like wires together in a cable) the radiated EMF will be almost zero.

The difference, way back when, was in the highs, where Class D was kind of grainy compared to a single-ended transistor amp used for comparison.