Full subtitles (aka Closed Captions if you live in the USA or Canada) in a game, can make the world of difference to deaf and partially-hearing players. Now that games and films have so many parallels, imagine how hard it would be to follow a film without the sound-track. It's not much a stretch to imagine how hard some games become without sound.
My friend, Reid Kimball, has posted a really helpful Mockup of Heavy Rain with full closed captioning. It shows some of the importance of having the mood of music described, the sound effects as well as the essential dialogue.
Reid is offering a service to advise game developers upon how to best include deaf and partially-hearing players. He definitely knows his stuff, after having worked with the team that modified Doom 3 to include full subtitles, and spending many years supporting the IGDA's Game Accessibility Special Interest Group.