for everones questions, i'm a sweeney todd freak, and i love everything to do with jD,TB,or HB, so yes they were singing on set, but they also had the songs recorded as well, because for if they were singing the entire time everyones voice would be very tired, and HB was pregnant during this film, so it was for the best, so it's kinda 50/50. i hope that anwsers everyones questions:3
ehhhh well firstly they record the tracks in a music studio at all diiferent speeds etc... depending on the movements of the film and what will happen for example in the worst pies in london that was recorded faster and slower to get the speed right for the film and then when they act on set they can lip sync when filming the music they recorded with orchestra is playing in the background but normally they sing to make it realistic with breaths etc...... i hope that answers your question a bit