Kotaku has reprinted an interesting letter from a deaf gamer asking the ESA to support subtitling in video games. The lack of subtitling means a game's story is often incomprehensible to roughly one percent of the population. Not only that, but audio-only cues are sometimes important to the gameplay itself, such as the beeping bomb-detection device in Metal Gear Solid 2.
What's worse, deaf and hard of hearing gamers often have no idea whether a game will cater to them going into a purchase. Game packaging rarely notes the availability of subtitles, and game reviews rarely mention audio accessibility (GameCritics is a notable exception, including a deaf and hard of hearing section in its consumer guides).
Game accessibility doesn't stop at hearing loss. Developers are working on audio-only games for the blind and The Switch Project supports simple, one-button games that "can be played by anyone, no matter the disability."