This is a good observation. I was wondering if anyone was going to ask this obvious question.

But what is heard in any voice, linguistically speaking, is neither the sound as an exterior stimulus (noise in the ears) nor a purely mental association, but the phoneme: what Saussure called a ‘psychic image’, neither a purely external sound nor internal idea. The inaugural gesture of modern linguistics is to open up this question of a kind of hallucination – a word or letter void of any possible reference but only contained in a system of differences — that is already at work in any linguistics that wants to call itself scientific.

The linguistic problem is similar as to when one ‘hears a ghost’, usually there is some kind of noise in the hallway, for example, but it is not a mere external stimulus, since the ‘noise’ or, indeed, voice, already has an internal ideation of the one who hears it. A formal linguistics necessarily includes the observer in the observed so that there is never simply reality to the exterior and ir-reality to the internal, rather they are mixed. This may be similar to what you are calling ‘clairaudience’, except I would claim it is not rare, but usual and normal today.

Freud called this mixture ‘psychic reality’ and proposed it is the basis of any study of psychosis.