How to activate clairaudience?

All spiritual knowledge that is conveyed to you directly from the second and higher spiritual zones reaches you clairaudiently, and by means of spiritual sound waves. And silence is necessary to the receiving and the recording of spiritual sound waves because these waves are keyed to a pitch so low that they cannot be heard amid the roar and clash of the sound waves of everyday life.

You must enter the silence to hear that still, faint voice with which angels speak, and with which spiritual entities of the higher spiritual zones seek to convey to you some knowledge of their world and life. Not only must you enter the silence, but in the silence, you must remain perfectly still. 

That is, you must keep your mind free from earthly impressions and in condition to receive the faintest sound waves that may be wafted to you on the wings of that eternal silence of which you for the moment have become a part.

The silence is the first essential. A mind free from earthly impressions is the second.

But before either of these can admit of success, you must have attained to a sufficient refinement of mind to make this organ especially sensitive to the presence of the finest and most faintly perceivable of sound or thought waves, and must have attained to a rather high state in your spiritual development. Otherwise, you will not get in rapport with any of the higher spiritual zones.

Fragments of Spiritual Knowledge Pertaining to the Spiritual World