This is one of those things I feel are belief based. If your thinking it strongly it would hold just as much "power" in my opinion. If your not so good at thought projection, then you should probably say it out loud :P
I think its a matter of what works for you works per se... But if you think about it magick is about trying to manifest your goals weather they be needs, wants, or desires into reality. Some say that sound travels in many realms as frequency touchs things in a vibrational way and in turn effects them for once it vibrates or is effected its no longer technically the same as before.
So when you think about it Words truely do have power because you can automatically aligne your mind, body, and spirit once you even start to chant or begin anything spritual for it signals a change from mundane to spirit also your words or gestures give a foothold for your magick to travel on into the physical plane, it also establishes the belief in yourself mind, body, and spirit needed to tap into a deeper source weather known or unknown, thats why spells are mostly repetative..
So when you think about it even when you invoke your asking the gods or whatever your invoking to fill you, or evoke asking for them to be with you, it comes from the well establishment of the words one speaks.
Obviously theres more advanced methods with using the mind but that will always be a one part to two. I mean you have it so why not utilize it even when added with thought, it'll just have it well established. So even a whisper is powerful.. Like I said its a foothold into the phsical plane.
Thats why people say be careful what you wish.. For its heard somewhere and it was given a foothold to manifest on the physical plane..
I hoped this helped and this is just one way to put it I'm sure theres many other ways to say it..