After hearing the floo again both Harry and Charlie had left the room, Harry more reluctantly than Charlie. Sirius thinks he might have passed out again while speaking with Harry; everything is so unclear. But perhaps that's just his head; just how he is now. People in high security wings of Azkaban don't leave them, and he hasn't heard much about recovery even from people who were only inside for a year. He's already come to terms with his wand being gone forever and the unlikelihood of him ever doing magic that isn't the animagus transformation ever again, perhaps it's time to accept that his mind – and the happy memories he knows he one had – are gone forever.
Sometimes a particularly loud voice will filter through the door and Sirius will break free of his dispare to try and understand; to listen, to help, to remember what he's supposed to be doing. He thinks things are going the way they should; that this is the best possible outcome for all the ways he's ruined things but no matter how hard he tries he cannot get a solid grasp on reality.
Desperate for things to make sense again, he transforms into Padfoot-Snuffles. Life is always so much simpler as a dog.