Curate, connect, and discover
Obviously if there is a bigger character reason for their quietness, not an issue
But if your sole reason for them being such is that they don’t speak please reconsider why you think that way!
Most of us who don’t use verbal communication as a main form communication (or are more comfortable being nonverbal) are not quiet! If anything we are the loudest people you’ll know!
It’s all about balance! If you have a character who doesn’t speak, evaluate the way they interact with their world!
Do they use sign language? Are there other people in their life who know sign language, and are able to engage with them?
Do they use a audio device like an aac?
If they had to get the attention of one person across the room in a noisy space, how would they do it?
Like every character you write, they should be 3d!! Just because they have something unconventional in their livelihood, does not negate this fact!