A mantra can be chanted in three ways:

1. By saying it aloud.
2. With muted voice, it shouldn’t be audible, but the lips and tongue should move.
3. Just in the mind, without moving lips and tongue.

Some mantras can be chanted in all the three ways and some in just the third way. So, it is always good to ask a competent guru how to chant a particular mantra.