All models of the Nintendo DS (as well as all 3DS models) include a microphone built into the hardware. Though the microphone can and has been used for voice chat in games such as Advance Wars: Days of Ruin, it is more often implemented as a gameplay input device. Games may ask the player to speak, sing, hum, whistle, or simply blow air into the microphone in order to produce a suitable reaction in-game.
As one example, the Nintendogs games are designed with extensive microphone use in mind. By speaking the name of the puppy and the names of the various tricks that the puppy can learn, the game will learn to recognize when the player is calling the dog's name or asking it to perform a specific trick.