There's no doubt that video games are popular. Sales are continually growing, with individual titles selling in their millions. Wii Fit, for example, has sold over 8 million copies while Halo 3 sold over 1.8 million copies during the first eight hours of release! Lord knows how many units it has currently managed, but one can be safe to assume it many millions more.
Now, out of those millions of sales, we can easily assume that the people who buy the games are not going to be identical. There are a high number of players out there who have some kind of disability. And video games should be as accessible as possible to as many people as possible.
One such disability, which is actually surprisingly common, is hearing impairment. Whether the player is completely deaf, or just has some kind of hearing issue, the game should be able to accommodate them -- and the simplest way to do this is through the use of subtitles.