Well, theoretically, you might try something along the lines of deciding how many actual lines there are, and spreading them out according to the length of the music. This would be fairly crude, because many songs have refrains, or repeats, or other timing issues that would mess this up.
You *might* try something like a hardware solution that would detect the beats of the music, and then display text after the correct number of beats has been detected. I don't see how you could do this automatically though - you'd have to use a different count for each song.
Something simpler might be to just allow the singer to request the words by pressing a button on the mic or something of that nature. But that would probably be kind of boring...
