I don't know how the crackling noise is made nor have I ever heard it. But: My theory at why it sounds as coming from inside the ear goes like that.

As a crackling noise we describe sounds with an extremely short duration following shortly one after the other with irregular intervals (Poisson distributed onset times most likely). We hear direction mainly by "comparing" (actually there are neurons tuned to do this work for us, namely neurons firing at maximum respectively for different onset intervals in both ears) which ear did first hear the sound and how long it took the other ear to record it as well.

If both ears are reached at the same time the sound comes from exactly in front (behind, above, below). If the interval is longest the sound comes exactly from one side (and other intervals between point to other angles. Some other processes that play no role in my explanation help you to tell whether the sound was in front or behind you.

One crack just sounds very much like the other. Imagine so many cracks in a short time that the intervals between them are in the order of onset time differences between the two ears.

For the sake of simplicity, imagine a series of 4 cracks in quick temporal order with irregular but very short intervals. The neurons in both ears signal eight crack onsets. The Neurons further up in the path (where the information of both ears is integrated), those that react upon onset differences, now run wild. The onset of crack 1 in the left ear and crack 1 in the right ear give one time difference (pointing to one direction). The onset of crack 1 in the right ear and crack 2 in the left ear give another time difference (pointing to another direction. Even the onset of crack 1 in one ear and crack 4 in the other may lead to a time difference compatible with still another direction of origin.

Now the binaural direction detection neurons run amok. Each of these many time differences points to another direction of the sound and all these directions are not compatible with each other. In addition to that the physical sound too may come from different directions. All sound directions sensitive neurons are stimulated and all of them fire close to maximum.

The "interpretation" for even higher up neuronal wiring is straightforward but confusing: The sounds come from all very quickly changing directions and not consistently from what our eyes tells us who does the chanting. The brain (Amos, look the other way) interprets this firing pattern as sounds originating in the ears, for noises coming from all directions often at the same time make no sense.

Add to that the atmosphere of Buddhist chanting, and the near simultaneous firing of neurons "reporting" all different directions leads to an impressive personal experience. It is a fine example of an acoustical illusion.

Wolfgang