The voice lines in ucn can really help prove Bvtoysnk in my opinion.
Though before that we got to also prove Will plush and Will speaker. In sister location we can see the plush in William Private room. We got the fact that the plush says tomorrow another day over and over again and they is a movie called tomorrow another day which two main characters are called William and Henry. Last piece of evidence Toy chica in Toy Chica the Highschool years who meant to represent William afton repeats some of the lines of The plush. So we know William the plush and since the plush in the final speaker speak in very similar way they are probably the same person.
In sotm we can see multiple flowers each meant to be a showing a part of the Afton family. The baby is Elizabeth. The grown woman is Miss Afton. The one which is laughing is Mike. The one we the missing teeth is CC and the grown man one is William and sotm we can see a scene we the grown man flower right next to the flower who teeth is missing who is lying down on hospital bed paralleling fnaf 4 and the final speaker. So as we can see William is the plush speaker and the final fnaf 4 final speaker but how does this prove Bvtoysnk.
Well many lines in ucn content back to what the speaker and the plush said to cc so why would any other character find a need to bring it up. Firstly nightmare Fredbear says the line We know who our friends are, and you are not one of them!" this contents to the line which Bill says in fnaf 4" We are still your friends" showing that the person in control of ucn has heard what William said in fnaf 4. It also proves the person does not like the speaker since they say that they are not this person friend. We also hear a lot of lines of putting some back together in ucn which pararels the final speaker telling cc that he "will put him back together". FOXY: “I can’t run like I used to, but I can pull myself apart just fine!”
MANGLE: “Now I get to play take-apart and put-back-together! You won’t feel a thing.”
NEDD BEAR: “This is how it feels, and you get to experience it over, and over, and over again. Forever. I will never let you leave.”
NIGHTMARE FREDBEAR: "Let me put you back together, then take you apart all over again.”
“Let’s see how many times you can be pulled apart.”
SCRAP BABY: “Let’s see how many pieces I can cut you into.”
“You won’t die, but you’ll wish you could.”
CANDY CADET: “He pieced the remains
So as we can see this a clear important thing to the vengeful spirit and being put together is already been show to be a theme in Cc story.
There also a lot of lines about teeths and mouths which makes sense for CC since he is the victim of the bite of 83. Come closer. Help me count my teeth!""You're not so big. Just a bite-sized morsel."- Nightmare BB. "Where is my beak? Lodged in your forehead, of course."- Toy Chica. Come closer. Let's smile together."_ with red chica."Let me show you how to break your face and look like me!- withered chica Now let's look at mediocre melodies. Firstly I believe cc parallels them both abandoned not love as much as the more impressive animatronics" We only just began I will never let you leave I will never let you rest" This parallels fnaf 4 of William bringing cc back to life not letting him rest. This is how it feels, and you will experience it over and over again, and again... Need Bear. It does not make sense for it be about death since William has already died so it makes sense for it to be about feeling helpless something Cc has felt and is now making his Dad feel. So as we can see all the voice lines help prove Bvtoysnk.
Sorry if this does not make any sense.
