My impression was that they were speaking in a sign language, and that they simply didn’t have the time or resources to animate that. Or they could have used cued speech, although if the game is set in the nineties or later I’d think sign language was more common, especially among younger speakers (cued speech is basically speech with manual hints to make lipreading easier, and nowadays it’s mostly used by older people, such as people who lost their hearing after learning to speak, so it’s easier for them to use fancy lipreading than to learn a new language).
Another way they could communicate, which might be a bit easier to animate, would be writing things for each other to read (but that would mostly be how hearing people could talk to deaf people without having to learn another way to communicate; the deaf people would probably still rely on sign language, or at the very least cued speech, since those are faster than writing).