It definitely does have difference, it is said that chanting mantras aloud (vachika japa) will take you more time & efforts to attain what you are seeking as compared to mental chanting (mansika japa).
Quoting excepts from the book written by Om Swami titled The Ancient Science of Mantras: Wisdom of the Sages:
Vigyan Bhairav Tantra states that there are only two types of mantras. They are, one, mantras that are internalized
(citta-yukta-mantra) and two, mantras that are uttered (dhvani-yukta-mantra). When it comes to chanting (japa), the mantras that are internalized are far more effective than those that
are chanted out loud.
Mantra sadhana, however, is an inward process. Chanting aloud like the
relentless chirping of a cricket does not take you closer to your
inner world.
In mantra yoga, all hymns, odes and eulogies to God or your deity can
be sung out loud as they evoke positive feelings of devotion and
celebration but mantra chanting is almost always done internally. It
takes a great deal of practice to chant your mantra internally without
losing your concentration.
Of all the types of mantra chanting, mental chanting is said to be the most effective way of chanting, but with concentration.
Mental chanting or mansika japa is when there’s no external
sound in your chanting and even your lips don’t move. A common
misconception is that mental chanting is chanting your mantra loud in
your mind. That’s not really mansika japa. The correct way to do
mental chanting is to recall your mantra with concentration, one word
at a time.
Mental japa is ten times more effective than the first type of japa.
It represents intellectual consciousness (pashyanti). It is important
to note that the sound it is associated with is called pashyanti,
means that which is seen (as opposed to spoken). In mental chanting,
you repeatedly visualize the mantra in your mind’s eye.
But main point to be considered while chanting mantra is the 'faith' with which you chant mantra.
