In general, copper tends to sound more warm, rich, and full bodied; which is why Dennis Had of Cary Audio loves the Jensen copper foil in oil capacitors for some of their tube gear. Copper can tend to fatten up, enrich, and flesh-out the sound in the mids and highs or tame the etched chalk-like sound of an aluminum ribbon tweeter or a titanium dome tweeter, etc.
Copper, electrically speaking, is slightly less conductive than silver, which is the best conductor due to its conductivity or lowest resistance per foot or given diameter, etc. Copper is second. Gold is somewhere down the list but is used for anti-oxidation purposes for signal transfer primarily and since it is also softer, it is more malleable and ductile to be slightly compressed to allow a tight fitting connection.
Silver, in general, tends to sound more open, airy, extended, light, and allows a greater higher frequency extension (treble) to pass through; all of which give the silver conductor a perceivable increased or greater detail as more high frequency content is being passed; especially upper harmonics. It can also sound hard, harsh, tinny, thin, hollow, weak, lacking bass or midbass or both, etc. This is largely due to the larger amount of high frequency content baing passed in comparison, or in proportion to let's say, the rest of the audio signal being reproduced.
I personally like copper overall. Hope this helps.