Mori was almost always silent. He didn't speak much, and that was a relief for Kyouya, who thought that with more people talking and making noise in the Host Club, it would be even more of a chaos. He was grateful that Mori didn't speak much.
Because he also knew for a fact that Mori wasn't always silent. He was intelligent, and he and Kyouya often had long conversations, just the two of them.
And during sex, Mori wasn't silent at all. He made the most sensual noises, that could drive Kyouya mad.
Sometimes, Kyouya wondered if Mori was so silent for him, so he would have less chaos to deal with. And Kyouya also quite liked that Mori only made noise when he was with him.