@yopy I don't think we're supposed to know exactly what was said yet. (Unless you can work it out from the picture if you know Japanese sign.) I imagine it will get revealed/discussed later. On page 15 you have the bit where the guy who gets punched and the two girls are giggling at something he says/signs which looks like a reaction to Taichi. Then the bottom right panel on page 19 is the bit where the guy that gets punched signs something that Kouhei and Ryu have a problem with. Its just my guess but it's surely something about Taichi's voice being quite loud or him just being quite exuberant. And it'll be a kind of situation where someone with hearing problems making fun of someone without hearing problems.
I just hope this back and forth of trying to protect each other and constantly misunderstanding each others motivations calms down soon. I like a bit of misunderstanding etc but this is driving me slightly mad. I know it's all about the enhanced difficulty of communication and I love how both are so considerate of each other and how they have their own problems and insecurities but if this carries on for another whole volume I might scream.