I'd long wanted to use recordings by other performers in my videos; the problem was how to align the timings in a MIDI file (from which the animation was generated) with the audio of a recording of a live performance.

I worked on the design of a tool that would partly automate this process, but did not complete it; instead, I developed a less automatic approach: record a click-track that's in sync with the audio, and write software that applies its timings to a MIDI file (described here).