First, let’s address the technical reality. Shutter Island has an incredibly dynamic audio range. One moment, you have the crashing of waves against the rocky cliffs of Ashcliffe Hospital. The next, you have Max Richter’s haunting string composition, "On the Nature of Daylight," swelling to drown out dialogue.
The subtitles visually change based on the tension of the scene: Calm Scenes: Clean, standard sans-serif font. Flashbacks/Delusions: shutter island with subtitle