Audio logs are voice recordings that are left behind by former inhabitants of the area the player is exploring. They are typically optional collectibles, but if the player finds them, they serve the purpose of filling in narrative elements without slowing down the action for the player, because they sometimes (but not always) allow the character to keep moving through the game as the recordings play. Many times there are achievements/trophies associated with collecting all the audio logs in a game. This is similar to the diary mechanic, but with audio rather than text.
Early examples of the audio log mechanic include Carmine (as a video log), the original Metal Gear (as an audio transmission), the Lunar and Chrono series (as video logs), System Shock 2, the Fatal Frame series, and Doom 3. However, it was Bioshock that largely popularised audio logs as a feature in games. Referred to as 'Audio Diaries', they were personal recordings by citizens of Rapture, and they filled the player in on crucial stories and characters within the Bioshock universe. Many of the audio logs were optional, but a few of them played automatically when the player reached a certain point in the game. After Bioshock, many more games, such as Dead Space, Red Faction: Guerrilla, Singularity, and Army of Two: The 40th Day, used audio logs in a similar manner; to fill in the player on the backstory of the world they are exploring.
Batman: Arkham Asylum uses audio logs to let the player listen to therapy sessions of all the famous inmates of the asylum, such as Killer Croc, The Joker, and Harley Quinn (among many others). Many of the audio logs feature characters that don't actually appear anywhere in the game, and instead just help fill in the fiction of the villains in the Batman universe.
In Halo 3: ODST audio logs were used to tell an entire substory within the game that chronicled the escape of Sadie Endesha from the city of New Mombasa, while being aided by a subroutine of the AI known as the Superintendent. This subplot also helped clarify some of the details in the main storyline of the game.