Is it necessary to chant the mantra aloud or can it be chanted in the mind?
Mantras basically, and that includes the Gayatri Mantra, have a certain sound effect apart from the meaning.
So, when you chant the mantra aloud, the vibrations of the mantra not only help us, they also help people who are around, whether they believe in it or don’t. It has nothing to do with this belief or that belief–it’s to do with the vibrations of the mantra.
Even if you chant silently–it has an effect on your own self. Gradually, your own self is so purified that you begin to help others, but that is an indirect effect.
Directly chanting the mantra audibly can help and affect people and other living beings that are around us.
As far as, meditation is concerned, chanting that is called Japa, is divided into three kinds–one, is the loud Japa or the loud chanting and then, there is the chanting where only the lip moves and then, there is the chanting in the mind.
While sleeping, you will dream that you wake up and see that you are chanting the mantra in there. This is not waking up in the real sense. I mean it is a kind of a dream in which you wake up and realise you are chanting the mantra.