It's a common error to treat the flow of electricity as analogous to the flow of water in a pipe; the smaller the pipe, the less water it can carry. A lot of the flowing water analogy works quite well, but it's not true that current through a wire is governed by the smallest piece of wire in the circuit, even though the flow of water through a pipe is a function of the smallest orfice in its path--which, of course, is why a faucet works!

In an electrical circuit, the effect of wire is proportional to its length. Since the length of the #18 wires in the speaker is short, it's effect on the cable resistance is correspondingly small. A one foot length of #18 wire is equivalent to 4 feet of #12. If you put a 1" piece of #24 wire in series with a #12 speaker cable, it will be equivalent to increasing the cable length by 8" . Practically nothing.