Maybe people concerned about having their voice manipulated can carry a device that generates an acoustic timecode whenever they speak, or at least some kind of non-repeating tone generator. Supposedly the 60Hz or 50Hz hum in recordings can be correlated with variations in the electrical grid frequency. It would make sense to go further and deliberately tag live audio with a timestamp to ensure continuity.

I can think of a few ways of faking a timecode like that, but they can be counteracted to an extent.