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At audio frequencies, it's usually irrelevant. When the transmission line gets to be longer than, say, 1/10 wavelength or so, then you start considering things like impedance matching and such.
To just get raw amps into the speaker, though, any wire that's rated for the amps will do - for speakers, they don't even have to be twin-lead - you could lay a couple of random pieces of wire on the floor, and the sound will be indistinguishable from the highest-end audiophool crap, unless they _insist_ on a certain amount of distortion, for example, to give that rich, golden toob sound.
Cheers! Rich