[jononor]

A vo-coder is probably the best off-the-shelf technology. Of course a challenge with making invasive changes to the audio (in order to defeat watermarking), is that people may claim that the audio is fake/misrepresented. Vocoded audio will not sound like the original speakers, and may have artifacts. Lipsync may also be slightly off. So one would have to be careful to communicate these limitations. Which the general public may not have much interest in understanding... Adversarial opponents may latch on to these things and use it to discredit the recordings.

An more conservative approach would be to transcribe the audio into text, and only offer the audio to (more) trusted parties for verification.