For years (15+) I have prerecorded live hqarmonies and simple backing rhythm arrangements to sing and play with. I never saw the need to sync, because they are real audio files playing through the PA as if they were additional band mates.
I used to use MiniDisks, then MP3 players, laptops, and now...my Korg PA800 does it for me internally !
I have the "Dave Boyd Five" with me at all times!