I am pretty sure it was posted on Slashdot (can't find it), but the Boston Globe reported in 2002 that scientists at MIT could convincingly alter video to make it appear that someone said something they didn't, with only 2 minutes of footage:
(Link to article on Boston Globe is dead.) They couldn't alter the audio convincingly, or at least didn't try. However, I also recall seeing on Slashdot (10+ years ago; also can't find it) that someone (Bell? MIT?) could take about 2,000 recorded words from an individual and create convincing audio of words and sentences not previously recorded.
Post-truth politics won't matter when someone releases convincingly altered video and audio of a public figure doing something that they never did.