There is no way to do this with no artifacts; it's just fundamental to how audio works. We have pretty good algorithms for it, but in the end you're doing psych

There is no way to do this with no artifacts; it's just fundamental to how audio works. We have pretty good algorithms for it, but in the end you're doing psychoacoustics to try to make it sound good. There's no mathematical model for doing this "perfectly" like there is for resampling. It's not hard to hear the artifacts in DJ tempo stretch algorithms if you listen closely, especially when they are applied to a full mix (as opposed to single instruments). Of course they're small these days, but when you're trying to measure a subtle effect, they're too much of a confounding variable.