Reported simply as “Richard from England”, one man has built his own plasma speaker, which uses tungsten-tipped electrodes and 60,000 volts of electricity to produce a large plasma arc.
A by-product of plasma arcs is ozone, which can be deadly in large quantities, but Richard reckons that the prototype produces none at all.
It looks brilliant, and probably sounds it (at least, for higher frequencies), but don’t expect to find it in your hi-fi system any time soon. It’d be incredibly expensive, and you probably wouldn’t get insurance any more.
More info on plasma arc speakers at Wikipedia.