There was only once when I heard a difference between wires to speakers.
It was around 1990 and specialized wire and cables was coming into vogue. The audio store I was at had three soundrooms. To satisfy my own curiosity, I rigged up a system in the lesser used one (the high end room) around an entry level receiver with a mono switch and a pair of Clements floor standing speakers. On the left channel I used the standard cheapo (silver one side, copper the other) speaker wire and Monster (or something similar) OFC wire for the right. Panning from left to right with the balance control, there was a definate audible difference. So much that I was instructed to rig up the same demo in the sound room that got the most use. It did sell better speaker wire for a lot of the sales people.
But going from one or two dollar a foot wire to seven/eight/nine dollar per foot wire, no perceptible difference.