Sound would be another topic, but I don't think sound is a must always (otherwise the deaf couldn't get satisfaction out of games.)
Instructions don't have to always be heard, they can be seen as well, so again, it depends on the game.
Do you think a blind person could enjoy a game? Yes. If the game was designed to be played using the sense of hearing, then it is possible even for a game to be played without having any visual elements at all.
See, games can go all sorts of ways, and can have all sorts of rules. So to answer whether something like that is important or not, one would have to consider all aspects of game design.
Imagine playing a game that intentionally denied you the use of one or more of your senses, now that would be interesting.
What about a game that denied you use of your sense of sound and seeing, only to rely on your sense of touch? Or perhaps on your sense of taste?