Thanks for the responses guys. I think I get it now.
Headphones run on DC in that the polarity on the wires does not change. And the current is always only running in one direction. Up through the L + R wires and back down through the shared ground. But there is still "alternating" happening, in that the voltage rises and falls rapidly to create vibrations in the headphone drivers.
So does the same thing happen in conventional speakers too? (Meaning that I was wrong about them being AC.) I've read some comments about powerful headphone amps being able to drive some speakers directly.
No, all audio devices transmit, amplify, shape, and or transduce audio waveforms. Headphones are transducers. They convert an electrical audio waveform/signal into sound pressure level fluctuations in the atmosphere. All audio waveforms/signals are AC.
Electrostatic headphones also need a DC bias voltage (typically 300-500vdc) present on the stators, which is analogous to the permanent magnets in dynamic headphones. The constantly fluctuating audio signal works against the constant magnetic or electrostatic force to move the diaphragm, and subsequently, the air to produce sound.
An AC signal, by definition fluctuates above and below 0 volts. In order for current to flow, and subsequently work to be done, takes two conductors. What is referred to as the "ground" circuit is really the return path for those electrons, The common ground carries the return path for both left and right channels. For the practical purposes of this discussion, it is not an issue to use a common return path.
Think of the whole affair as two water wheels, each one being turned independently by independent streams of water fron two tributaries. As the water flows over the wheels, producing work, the run off from each wheel is dumped into a single larger stream that has the capacity to carry off the runoff from both of the smaller streams, without backing up.
If there were no place for the water to go from either stream, no work would be done and we would have a flooded mess on our hands. ;-)