Well, the naive way to do it would be to detect the onset of each beat by assuming it corresponds to kick drums on fours, and then speeding up or slowing down t

Well, the naive way to do it would be to detect the onset of each beat by assuming it corresponds to kick drums on fours, and then speeding up or slowing down the track you're mixing into to correspond with it, timing it so the fours line up. This would be very easy to implement and over a longish crossfade (say a couple of bars) would be fairly convincing. Using AI you could probably intuit the "pulse" of the music to find the first beat of the first bar of a measure, and sync tracks up so that they mix in a place that makes musical sense.