The Westminster Chimes that play when 6 AM rolls around followed by the sound of children cheering. Especially if it comes right after Freddy's tune, since you've been literally instants away from death.
Ironically, Freddy's tune can be this without context, especially given that it also plays during the ending screens. Even stranger, on 4/20 mode, most consider it a blessing, since during the last digits of power, players sit there to conserve power, but run the risk of Foxy bursting in. Markiplier, for example, let out tense screaming out of fear before letting out a brief cheer when the power ran out.
The same Westminster Chimes and children cheering as the first game, when the clock changes from 5 AM to 6 AM.
The ending theme. It's especially accompanied with a picture of Toy Freddy, Toy Bonnie, and Toy Chica in their 'peaceful mode' and not scary, so they look quite adorable... somewhat.
Circus Baby's voice may qualify, as it's genuinely soft and soothing.
Some people have too much fun with the various nose honks.
But most people enjoy the simple tune that plays every time you finish a night where your shift is complete and you enjoy some time at home, including the one where if you want to get the fake ending in Night 5.
Beating your high score earns you a rather cathartic jingle.
The industrial fan-like noise in the Old Man Consequences easter egg is weirdly calming if you know Old Man Consequences is actually a rather nice guy and isn't out to hurt you at all. And if you've had it up to here with William Afton's shenanigans throughout the series, speeding the sound up gives you the audio of him screaming in pain and terror for help that will never come to him. This one also counts as a Catharsis Factor.
If you're the kind of person who relishes in the more random aspects of the game, Dee Dee coming in to give you something new to play with can be this.
There's something strangely cathartic about using a death coin and hearing the "thunk!" sound play. It's gratifying to be rid of a particularly annoying threat for the rest of the night, especially coming from your own hand.
Summoning Rockstar Foxy and hearing his friendly voice, especially if he offers something that can win you the game, such as a soundproofing. That is, unless you hear "Ya win some, ya lose some", which can invoke the opposite reaction.
If you're a part of the fandom who believes in the theory that this is William's personal hell, the animatronics chewing the player character out for their actions turns from annoying to this.
The screaming in Sister Location, typically reserved for jumpscares, becomes this in Ennard's Vent Repair level when Ennard shrieks in pain as you incinerate the animatronic alive; a fitting way to end one of the only times in the franchise that has you actively fighting back against your robotic tormentors.
that play when 6 AM rolls around followed by the sound of children cheering