I'll post more info about the beat-sync soon (hopefully - there's a lot going on right now). If you want "perfect" beat-synced videos, then you'll want to use the Beatgrid feature of Scratch Live (or Itch) to analyse your videos, and save the Beatgrid to your video file. If that isn't found, then MixEmergency can fall back to using the BPM of the video, synced from the first cue-point that you have saved.