She's deaf and mute. Her deafness wouldn't inhibit her from crying, but her muteness would. She can still tear up I'm sure, but it wouldn't be the same vocalized crying as a normal person.
Shizune is not physically mute. She simply chooses to be and has good control over her voice. She can and does make the odd noise (notable examples are during her sex scenes and she audibly laughs a few times even if she is quiet), but when she's aware of that noise she stifles it.