Jim,
Electric current is a flow of electric charge. Electric current produces magnetic field around the wire. Any change in amount or direction of electric charge produces a change in direction and amount of magnetic flux around the wire. Change in the amount of electric charge that flows in the wire, as well as amount of magnetic flux that follows, both change at the fraction of the light speed.
Presence of the load creates electric field between wires. Both electric and magnetic fields are necessary to transfer the energy from source to load. Both fields together create energy field (Poynting vector) toward load (that absorbs it), that follows changes at the same speed.
In case of the current thru the fuse, that was mentioned, energy field will be traveling thru all the space between fuse and the return wire. The further they are the larger this filed will be. It is wise to keep them close to reduce it.
As for the purity of the metal - the obvious thing is resistance of the wire, that causes power loss, less obvious is inductance of straight wire, affecting wire impedance, especially at higher frequencies (inductance is smaller for thicker wires). Second aspect of purity is that impurities in the wire reside between crystals of metal. Typical impurity of the copper is copper oxide (that is a semiconductor). What is audible, and what isn’t I don’t know. Often it is a placebo effect but I don’t mind. If placebo effect works it is worth the money :)
As for silver wire sounding different than copper - I have no idea. The obvious difference is lower resistivity of silver, but it is a very small difference. Silver, according to many, produces brighter sound. Maker of my cables, Acoustic Zen, adds few percent of copper into silver. To me it is all on the verge of black magic.
