Water, and air would also work. My guess would be air for physically stopping sound from emanating from you (like Hisako's light spell that stopped her hands from giving off light when casting) and water for a radius type spell giving the illusion that your not making sound to beings close by, might not work on animals /higher beings, a-la his mist spells
I suspect that both Air and Dark would have Silence Spells, possibly Life but that would likely stop someone from speaking rather then creating Silence.
