True, essential to karaoke is that the lyrics are presented roughly in time to the music, but that could certainly be done automatically as well -- in fact, it's more or less how the follow-the-bouncing-ball animations from the 1920s on were created, although in the earliest days it was easiest to work the lyrics slide by hand and use a baton for the ball. It seems like this could have been mechanized by a sufficiently clever design.