Audacity, the open-source audio editing application, comes with a whole toolbox of effects that modify vocal recordings. By layering these effects you can achieve an almost unlimited variety of vocal distortions, from a robotic clang to a satanic rumble. Audacity lets you preview these effects without having to go through a lengthy rendering process, so you can adjust the effects settings by ear before you commit to them.
Audacity's "Change Pitch" effect, found under the "Effects" menu, allows you to shift the pitch of a voice up or down without changing the tempo of the recording. Frequency changes greater than about 8 per cent definitely begin to distort the voice. For instance, shifting the pitch up 15 to 20 per cent can make a male voice sound distinctly female. Shifting the pitch down 30 per cent gives the voice a definite demonic bass.
Audacity's "GVerb" effects give you many ways to adjust the reverb on a vocal recording. Setting the "Room Size" up to 200 or greater and giving the reverb an "Early Reflection" level of around 6 dB can make the voice sound eerily hollow, as if it's coming from the inside of a tunnel.
The "Wahwah" effect is designed to give a wah-type distortion to instruments like guitars, but used on voice it can give a science-fiction type distortion. The distortion is most pronounced when LFO Frequency, Depth and Resonance values are set up near their highest possible values.
The "Vocoder" effect maps the voice in the left channel onto a "carrier wave," which can be any kind of continuous sound in the right channel. You can create sounds from the "Generate" menu to create different carrier waves. Using a white noise carrier wave makes the voice sound coarse and static-filled; using a sawtooth tone wave makes the voice sound alien.