I looked in Google for Plasma Tweeter Circuit and found it. Simple Plasma Tweeter
Its TV flyback transformer is made for only a single frequency so it wll not have a flat frequency response with audio.
Here is the very simple schematic. C2 is a whopping 470uf but does not need to have such a high value. Its polarity is shown backwards.
Two power supply bypass capacitors are missing.
The text says the audio modulates the frequency which is completely wrong. The datasheet for the 555 IC shows that it produces pulse-width-modulation that varies the average output voltage and therefore the power of the plasma arc.
So when the music gets louder then the arc also gets louder. The frequency is the high frequencies of the music.
It is a single frequency oscillator. But music has many frequencies. I think in some weird religions they sing a single frequency drone.
It also cannot be modulated for changes in loudness. But music has many changes in loudness (except acid rock "music" that is always as loud as possible).