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, quoted in 'Gems from the Tantras' by M. P. Pandit)

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