A certain well-known speaker manufacturer that I favor uses Monster Cable inside their enclosures. Fortunately, they've had the good sense not to market this too much, as the use of Monster Cable is a two-edged marketing sword. Some people would say that's great while others would go out of their way to criticize it in another pointless debate.
As DMF says, there's so little wire in there that it's not much of a factor when compared to the longer runs from the amp to the speakers. However, most quality speaker manufacturers use decent wire. It may not be brand name stuff, but you can be sure the engineers spec'd out wire that meets or exceeds the performance of most brand name cable.
Now on cheap, poorly engineered or parts bin speakers...
