Speaker cables would be a very poor antenna. Since wavelength of 500kHz switching frequency is 600m a typical 2m cables would be 1/300 wavelength antenna. Usually antenna becomes rapidly ineffective below 1/10 of the wavelength.That's true, but in order to meet EU directives, UL and a host of others, the speaker cables are effective enough that the issue has to be addressed by any classD amp producer. In fact it is speaker cables that are mentioned when the issue of HF radiation is brought up in the design papers.
However, its so easy to address that I regard it as a non-issue. As I do the switching noise of SMPSs. The only reason we don't use the latter in our OTLs is the cost of custom SMPSs- its a lot cheaper to use traditional supplies!