Using old speaker cables for an IC

Also RCA recepticles could be converted to speaker binding posts.
You also need to be creative to find proper RCA plug that would fit certain gauge of wire.

First of all, mixing copper and silver is fine and in fact quite common. The vast majority of cables made have brass ends with gold plating, and brass is a much poorer conductor than copper or silver.

As far as the speaker wire to IC conversion, yes it's possible. You'll likely have a hard time fitting the wire into your RCA connector, but you can make it work. You'll probably have an easier time and a better sounding cable if you just try to make one from scratch.