The Korg PA800 plays midi or MP3 perfectly, and they can live on the same USB drive. I know Fran C had some trouble with his, but he had a broken keyboard. Mine is 100% reliable, and has never so much as glitched in performance. I use premade midi files, custom MP3 trax that I made and commercial (audio only) backing trax, and I mix them all in with my live parts in whatever blend is appropriate. I have some doo wop files that are just me with a harmonizer as the backing, and I have some full arrangements that feature my backgrounds so I can sing different lines eg: Drifters, Beach Boys, that type. I recommend putting background vocals on any trax that you make - it fills up the arrangement so well, and gives the people more of the thing they can closely identify with: WORDS! You may not have many musicians in the crowd, but most people (think they) can sing.