In C is Terry Riley’s 1964 score for any number of musicians to improvise through 53 melodic fragments at their own pace, so no two performances ever line up the same way. iNCYNC lets you stack recordings on top of each other — a 1968 Riley session against a 2024 Kiyomizu-dera temple performance, say — and nudge each one’s playback rate until their pulses sync. A “Play Along” mode adds the 53 fragments yourself, in a separate window.