I think it's mainly personal preference. You are the one casting the spell and spells/chants help you to focus on your goal so I see no reason you cannot do so silently, within your mind. I personally vary depending on the situation. Obviously it's better to have a quiet, private place when casting so your mind is centered & focused but it's not unheard of to cast within earshot of others. If I think someone might overhear I cast silently but if not then I might cast verbally. Depends on the environment, the mood, and perhaps even the spell itself.
Re: Is it necessary? By: Lark Moderator / Knowledgeable
Post # 5 Mar 27, 2018
Spells do not necessarily involve spirits. They can, if that is what the caster wishes to do. But many of us simply use our own will and the energies all around us to cast the spell without getting spirits involved at all.
@SindAR16, you can certainly chant the words to a spell in your mind without ever saying them out loud. Many of us have used this method with the same chances of success as if we were to voice the words.