Things you can do to speech and still be comprehensible



Modify the voice quality to make it softer or more metallic.

Maintain constant voicing (eg with those touch-the-neck-when-speaking doodads).

Modify the pitch as you speak (singing). Talk in a very high or low voice (but some vowels are harder to pick when singing very high).

Modify tempo while speaking, eg drag out a single syllable for a long time (Dave Hughes), or double or halve tempo in the middle of sentence.

Remove pauses. Add pauses. Add "ers" and "ums".

Pinch your nose shut.

Modify the volume of a recording.

Modify the speed of a recording considerably (Alvin and the Chipmunks) (note that the physical correlate of this would not be a change of pitch, but scaling of the entire vocal tract).

Add considerable background noise.

Add considerable echo and/or resonance (eg in a room with reflective walls).

Whiten a recording using short time LPC, shuffle whitened samples within each frame, unwhiten (sounds whispered).

Knock out large chunks of the frequency spectrum. No part of the spectrum is essential.