Cute! It's pretty much the same as the Cassius app from a few years back (https://itunes.apple.com/en/app/cassius-i-3-u-so/id399394777), but syncing to external audio instead. A nontrivial extension, but still very similar from the user's perspective.
Now, if the app was able to lipsync to any tune, that would be more impressive. It's certainly doable, see e.g. Tony Ezzat's "Mary 101" system from 10 years back: http://people.csail.mit.edu/tonebone/research/mary101/