Murph, joining different sizes of wire creates no audible problem; this isn't analogous to a multi-lane superhighway suddenly turning into an 8 foot wide dirt road. The signal that drives the speaker is actually carried in the electromagnetic wave that travels along the outside(not inside)of the wire at nearly the speed of light. The part of the wave energy that penetrates into the wire and latches on to the electrons to guide the wave along the wire is lost as heat(the amount depending on the resistance of the wire)and doesn't directly contribute toward the sound in the speaker. The electrons in the wire move very slowly and if this is what really carried the sound it would take an hour or so for it to reach the speaker.
A good explanation is found at
pp.2-3 of a section of the materials which Dr. Lesurf originally prepared as an introduction for his students in electronics and physics at St. Andrews. Keeping the resistance of the length of wire low to minimize the amount of wave energy drawn into the wire and lost is desirable, but for example, joining a short length of 16ga wire to a 12ga wire might result in an overall net gauge of about 12.1, and isn't significant.
