I have very successfully made speaker cables out of interconnect wire, to be precise Canare GS6. It is Sold by Canare primarily as guitar wire. It is made with a single stranded 18 awg OFC conductor in a teflon insulation and a braided copper shield. Works great for 3ft long speaker cables.
I am using the same wire for an interconnect, between my processor and pre-amp. This is not critical application in my system as pro is only used to decode the Dolby from TV into analog. But I must say other wires by Canare such as L-2T2S make a better interconnect.
So if you have a not so great interconnect wire you may be able to make speaker cables from it, but I doubt the opposite is possible.
