Audio problem: Right after Joseph Buquet is killed, Raoul runs and finds Christine. When she says, "Raoul, we're not safe here", her lips do not move; her mouth is merely agape. (01:05:35)
Audio problem: During the end of "Prima Donna" when everyone sings "Light up the stage." you can see some actors and actresses singing the word "light" but you don't actually hear it until a few seconds later.
Audio problem: When Christine and Raoul leave the lair, singing to each other, she turns her head back toward the Phantom as we hear her singing 'each night, each morning,' but we can see she isn't saying anything: her mouth and lips aren't moving. It can't be that the Phantom is remembering the rooftop scene, because you hear the voices echoing in the cave, resonating differently as they move away. This is confirmed by the screenplay (p. 140 of "The Phantom of the Opera Companion"), which states that the Phantom hears them singing in the distance. (02:09:40)
Audio problem: Several times during "Masquerade", starting with the very first shot of the chorus singing, the audio does not at all match their lip synchs. (01:18:10)
Audio problem: During Christine's lines in her first scene of the film, her lips don't match what she's saying. Her lines must be added during post-production.
Audio problem: When the Phantom takes Christine down to his lair the second time, in the shot where the camera is behind his neck and facing Christine, you can see that the movements of his jaw are out of synch with the words he's supposed to be singing.
Audio problem: In the opening song of Don Juan, Carlotta obviously cuts early, although her voice is still heard hitting the high (and obnoxious) note. (01:47:20)
Audio problem: Right after Firmin says "It would appear they've met before." Andre replies "Yes." However, his mouth and the words don't match up. (00:26:25)
Audio problem: In Prima Donna, when Carlotta is on the bed that Andre and Firmin are carrying her on, the words "Prima Donna once more" are sung but Minnie Driver, Carlotta, "sings" the words after the actual words are sung.
Answer: I like to think that he would always place a rose there, possibly every week or so, and they never had the ring on them. And he knew When Raoul would be there, so he placed them when he knew he would be alone with her. As for the two years later, since the phantom "knew" when Raoul came... he especially put that ring there. Just to let Raoul know about his love for Christine and the fact he is still living. He knew Raoul would visit then, and knew what to do. (This is only a theory I have proposed. As for a genuine answer... I am not aware of one.) Hope this maybe helped.