There's an interesting article up on the Boston Globe Online regarding a small company called ZForm, who's aim is to level the game playing field for persons with sight impairments. Much of the article discusses their sound-only modification of Quake which has allowed a blind friend to frag at par with the developers themselves.
To make Quake work for a blind player, the team had to create a world of aural cues that evoked the complex visual world of Quake. When a hallway comes up on the right, for example, the player can hear a slight rushing of air off to the right. Other things in the game - objects like guns or competing players - were programmed to give off characteristic sounds, their direction indicated by their relative volume in the ears. For their version, a pair of headphones are important.
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