I built a plasma speaker recently with a flyback transformer, no gas or anything. It does produce a little ozone but a lot less than my air purifier does! When the arc is stable it is hands down, the best tweeter I have ever heard in my entire life! It also has 360 degree sound so you can put it in the middle of the room and the sound will be the exact same wherever you are. The volume depends on how many volts you put through the flyback so 12v dc is pretty quiet but I put 19 through mine before the MOSFET got too hot and I could hear it from upstairs. I ordered better MOSFETS so I can put at least 40v through the transformer. Yes, you can make a fullrange, 'perfect', speaker with a plasma arc but the arc would have to be over a foot in length to vibrate the air enough to produce anything lower than 100hz. That probably would produce a lot of ozone and take up a lot of power. Helium gas is used to negate ozone; it doesn't make the arc any better. It would be much easier just to add a subwoofer and midrange speaker to the final design, I'm looking to make a 2.1 system like this, I will let you know how it turns out.
Hope this helps!