alt Hacker NewsThe classic fable round these parts is Quad (and/or Cambridge Audio?) demo-ing their latest and greatest at a 1970s Heathrow Expo using mains cables as speaker wire.
It’s the least important part of any system and indeed my Quad amp and CA R50s are wired with twisted, braided, brown lamp cable as a nice aesthetic homage.
Why wouldn't mains cable make good speaker wire? Probably much larger diameter than needed for audio and therefore more expensive if fairly priced, but if you've got to wire speakers and that's what you've got, should be fine.
About the only things you could do wrong would be using wire that's too small to carry the load, is frayed/broken/severely corroded, or is coiled in a way that inductance becomes a real issue. Running parallel and near electrical or signal wires is problematic, and largely different run lengths can make a difference.