This study examines the functionality of sound in computer games, a topic which has received little attention in game studies and media studies. It is a study of the communicative aspects of sound in computer games that utilizes theories from both film studies and research on audio as a system of information in computer-based and physical environments. This work focuses on the relationship between game audio and player actions and events that take place in the game world, and shows how sound in games support the player's interaction with the game. The study argues that computer game audio brings together the sense of presence in the game world with a principle of usability. The study finds empirical support in interviews with players and designers of games, as well as in-depth analyses of the real-time strategy game Warcraft III (Blizzard 2002) and the stealth-action game Hitman Contracts (Io Interactive 2004). These are two games that represent different ways of using audio for informational and atmospheric purposes.