I believe twists are usually put into cable pairs to prevent crosstalk. If you look at the 4 twisted pairs in a Cat5/5e cable you will notice that each pair has a slightly different tpi (twists-per-inch) ratio. This further cuts down on crosstalk between the pairs. Not sure if this is relevant on speaker cables where there is one signal per cable pair, especially since they are not (I hope) bundled.

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