OM is the mantra, the expressive sound-symbol of the Brahman Consciousness in its four domains from the Turiya to the external or material plane. The function of a mantra is to create vibrations in the inner consciousness that will prepare it for the realisation of what the mantra symbolises and is supposed indeed to carry within itself. The mantra OM should therefore lead to

wards the opening of the consciousness to the sight and feeling of the One Consciousness in all material things, in the inner being and in the supraphysical worlds, in the causal plane above now superconscient to us and, finally, the supreme liberated transcendence above all cosmic existence. The last is usually the main preoccupation with those who use the mantra.


page 745, Letters on Yoga , volume - 23 , SABCL

Both of these [OM and the sound of church-bells] are usually sounds that indicate the opening or attempt to open to the cosmic consciousness.


page 1083, Letters on Yoga , volume - 23 , SABCL