Miki is actually mute; Miki can not speak.

Miki is a bookworm, often isolated and very lonesome and has been mute ever since she was 10 unknowingly why by the other Elemental Girls (Disincluding Kasumi) - Reason being was fear and trauma.

Kasumi often attempts to encourage Miki to speak, hoping her voice would come back.

Miki can write, so she often writes to communicate with the others if needed.
Miki can also blow air in which can cover objects/people in ice.