Is there a way to distort a sound so that can't be restored to its original form (i.e. the function moves one way), but it doesn't add noise to the sound (you can still decipher certain features of the sound based on listening to it; no randomness is added).
You should investigate a linear predictive coding mechanism like Silk or Speex. Encode the speech with said codec then decimate the bitrate down to the minimum possible intelligible level. I defy anyone to reverse the processing.