How does audio get transmitted from my iPod to speakers?
If you look at it from the standpoint of the wires going to the speaker, the voltage and current are both fluctuating, in a ratio given by R=V/I.  (The speaker isn't a true resistor, but close enough for this explanation).

What you've described, holding the voltage constant and varying the resistance to produce a varying current, is actually the job of the transistor(s) inside the music player.  Again, probably not that simple inside your iPod, but hopefully you get the point.