I got a plasma speaker kit from ExcelPhysics, and had some fun soldering it together. It turned out pretty well! Here is a short video demo:

Some details about the hardware:

• A 555 timer IC is used to generate a high frequency carrier tone, to which audio modulation is added
• A flyback transformer creates high voltage plasma arcs and is driven by the carrier and audio signal
• Input power: 12 V, 5 A