Class D Technology

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.

Talking about Spread-Spectrum during cold war could put you in a prison (top secret).  Today all CDMA cellphones (Verizon, Sprint etc.) use spread spectrum.  It is interesting application for class D, especially when receiver reacts only to average value (no keying needed).

There is about 1% of switching voltage noise on the speaker cable

You need to rethink this.
Look at the switching noise ringing on the test square waves Stereophile show when they don’t use their special Audio Precision’s auxiliary AUX-0025 passive low-pass filter to hide the switching noise ringing from view.

This is what’s on the speaker cable on the $7K Anthem Statement M1 monoblocks, and it more like 20% of the wave form.

Without Audio Precision’s auxiliary AUX-0025 passive low-pass filter
http://www.stereophile.com/images/1212AM1fig02.jpg

With Audio Precision’s auxiliary AUX-0025 passive low-pass filter
http://www.stereophile.com/images/1212AM1fig03.jpg

Cheers George