However, the highlight is clearly the elaborate system for breathing sounds. The frequency and intensity of breathing changes with a character’s activity, as you might expect, but Naughty Dog also distinguishes between open- and closed-mouthed breathing, injuries, stealth and vertigo. The game blends and transitions between these sounds, and there are custom sounds for emotions in certain scenes, like fear or exhaustion. Breathing even influences the facial animations.
You may sometimes be too caught up in the game to notice the effects. To some degree, though, that’s the point. The goal is clearly to produce realistic sounds (insofar as you can in a fungus-based apocalypse, anyway) that keep you engrossed in the story, and that can include subtle cues you might not always notice.