Cruising around YouTube, I found "Plasma Speaker/Singing Arc," an intriguing little video of a homemade "speaker" that makes sound by directly ionizing air.
It's not exactly high-fidelity, but it shows there are still a few tinkerers out there thinking about something other than the iPod.
The person behind the video describes the process: "This is a prototype of a FM modulated plasma arc speaker/tweeter. Have since built this circuit on a custom PCB & made an improved vertical discharge setup, using tungsten-tipped electrodes (see my other videos). This stops the plasma hopping about and causing the distortion you can hear..."
Back in the early 1980s, there was a high-end speaker using this technology, the Hill Plasmatronic. (It sold for something like $8,000 a pair.) The plasma tweeter was mated with conventional midrange and woofer drivers; the treble from 700Hz up was produced by a ball of ionized gas. The massless tweeter was hailed as state of the art and has never been surpassed. I listened to a pair of them back in the day and mostly remember loving the tweeter and hating the speaker. You can find used Plasmatronics every now and then on the Web. Definitely a cult item.