So you would like to build your own Stormtrooper Voice Changing helmet? SparkFun has you covered. Their Vox Imperium guide shows you how to build an in-helmet voice modulator. You are going to need a Teensy 3.2, speakers 1Ah battery, resistors, and a few other components to get started.
Those of you familiar with Arduino or Raspberry Pi are going to have an easier time building this on your own.