In all my years in recording studios, and in over-the-top higher end home audio/theater settings, I don't think I've ever been in a situation where a speaker wire was made even warm to the touch. Unless you're trying to push many hundreds or thousands of watts at rather high amperage to a speaker, I find it pretty hard to believe that you could melt even cheesy zip cord with an audio amp.

I agree with Gonk that even at double that temperature range it probably makes an insignificant difference in the conductivity of the wire. Nothing to worry about.

Cheers,