Call of Duty: World at War, released in 2008, introduced Zombies as a cooperative survival mode where players held off waves of undead enemies, and the round-end sting became one of its most recognisable audio cues. As gameplay videos, montages and streams spread online, viewers learned to associate the sound with clearing a wave and preparing for a tougher one. It remains in use because that association is immediate for longtime players, and because short gaming clips still borrow classic mode sounds to signal tension, relief and progression.