You're correct! Rifftrax will still occasionally do an MP3-only riff that you need to sync with the movie, usually for popular movies like Twilight or Dune that

You're correct! Rifftrax will still occasionally do an MP3-only riff that you need to sync with the movie, usually for popular movies like Twilight or Dune that they could never license. They have an app that's supposed to automatically sync with the film; I assume it listens for the opening sounds of the studio logo.