My sister loves music and I was rigging up my old system at my hers a few years ago, she wanted the thinnest wire possible, she's houseproud so didn't want ugly cables. I bought some single core bell wire from Wickes in an attempt to deliver her wishes... it was useless, no bass whatsoever. Wickes also did white speaker cable so this is what was used. I have no idea what the wire gauges were but the bell wire in this instance was utterly useless.Even 20 gauge is good for >20 foot into an 8 ohm load.
I have some quite expensive speaker cable here, which I purchased many moons ago, when I was, shall we say, more easily convinced. I do still use it, but I performed sweep tests on different speakers using different cables, and it turned out it was no better than a length of flex, cut from an old electric lawnmower.