About the use of mantra and its repetition ('japa')

"Japa is of three kinds.
Japa done aloud is the lowest;
Japa done in low tones is the middle;
Japa done mentally is the best."

- from the Kularnava Tantra

Hear the class on mantra here:
ॐ Om/Aum - The Primordial Vibration (clip from Class 2)