The Sims in Sims 2 (and all the expansions) speak "Simlish," which is a gibberish language. There is no need to hear it because you can tell what they're talking or thinking about based on the thought bubbles above their heads and other graphical representations. In fact sometimes the Simlish is misleading because a character will sound angry, but it turns out she's happy (you see little plusses or minuses over their heads representing how the conversation is going).
When important events take place in Sims 2, there is a little camera that zooms in on the appropriate place, like a burglar sneaking in or a fire. Also when the phone rings, it shakes visibly on the wall. When things happen like a good party took place, or a promotion, it's all done via written messages, not sound. I can't think of any situations (although there might be some) where you have to be able to hear in order to participate in the game. It seems pretty deaf-friendly to me.
