The idea of 100V speaker systems is to make it possible to distribute audio all over a large building with minimum loss of signal. Are you aware of I^2*R and how it is used to reduce transmission losses on long power distribution circuits? It's why electricity grids operate at high voltages. Same principal.
A 100V speaker needs an amplifier with a 100V output, except that, in my experience at least, 100V speakers are normal low impedance speakers with a transformer to match them to the 100V distribution, so, if that's the case with the speaker, you just have to remove the transformer and use a normal audio amplifier. 15W class D amplifiers are available on your favourite retail web site for a few £, $ or whatever the currency is in you part of the world.