Deku became deaf a long time ago from one of bakugous attacks in bullying him, he cupped his ears and did explosions into them damaging pretty much everything. Bakugou is still apologizing to him to this day being a much better student and hasn't been an asshole, well he still is but just not to deku... kirishima was born mute and he met deku and bakugou staying in a group with them because they all knew sign language and loved to talk to each other and no one knew what they were saying. HAHAHA