There's no metallurgical reason (or other)why one metal or conductor will sound different from another metal or conductor, unless in the construction method they've changed the LCR values......
Cables are very much like Swiss watches (and I like Swiss watches btw) they can be bloody expensive but don't work any better than £10 Casio's (in fact worse..)
Ifyou cannot measure it, I agree there should not be a difference, however, I have heard an EAR 509 rewired with silver wire inside and it sounded brighter tizzier with less bass afterwards. Whether this was the wire or the soldering or something else I do not know. But what is certain is the most conductive of all metals are silver, copper and gold in that order
. Silver is also the most thermally conductive element, and the most light reflecting element. Silver also has the unusual property that the tarnish that forms on silver is still highly electrically conductive.
So now that we know Silver is the most conductive - technically it should be the ideal carrier but as we know it is so expensive that copper is generally used as it is not far behind and is far cheaper.
Technically, therefore, cables made out of silver shoud be superior and can be measured to be so through their better conductivity. It would also account for the sound differences when silver is present.
Because of its expense it is relatively thin cable with only a few strands and I have not had the chance to audition a 200 plus stranded silver cable to see how it compares with a copper one of the same proportions.
I am convinced that in the amplifier the high end and tizziness was caused through thinner cabling and cable routing which was shortened to cut costs by the Japanese person who did it but perhaps Tim the designer might comment on that one some time.
But is does to some extent show that Silver can sound different to copper. To test the theory with loudspeaker leadsyou need to use copper wires of the same proportions to compare against the silver ones.
Many top end tone arms now use silver wire but that is because the lenght is short and the profile thin (and from what I can see they use the same amount of metal as a copper wired one).