Silver is only about 7% more conductive than copper, which is a measure of how much electricity is lost as it travels from point to point. Silver looses efficiency over time due to oxidization so it is typically used in specialized sensitive equipment were high conductivity over short distances is required. Gold actually has lower conductivity than silver or copper, but the differences are insignificant in audio cable, and as mentioned above, gold does not oxidize so it is ideal for plating. For an example of just how minor the difference is between silver and copper, a 1,000 foot long 24 gauge copper wire will have 2 ohms more resistance than silver.