Repetition of a mantra is called “japa.” Japa is essentially “mulling on a statement.” To “mull” the statement, we repeat it. Each time we repeat it, we “mull” it / consider its import and meaning.

This repetition is like chewing food. the more it is chewed the easier its nutrition is released to the bloodstream.

The mantra can be repeated verbally or silently, both are considered “japa”. Think of the two approaches as analogous to chewing with the front teeth vs. chewing with the molars. At first we tend to bite off a chunk with the front teeth, later we grind the bite with the molars. Similarly, japa tends to start with an audible statement of the mantra, and gradually proceed to silent repetition.

You can alternate the two approaches, to “mull” the mantra more effectively.