With regard to the validity of the theory that auditory hallucinations are, in part at least, a product of subvocal (very quiet) speech, there is a 1990 paper (available from ResearchGate if you create a free account)
Green, M. F., & Kinsbourne, M. (1990). Subvocal activity and auditory hallucinations: Clues for behavioral treatments?. Schizophrenia Bulletin, 16(4), 617-625.
which concludes (regarding the theory)
“In summary, the studies that have considered subvocal activity and auditory hallucinations have often used very small sample sizes, poor or absent diagnostic procedures, relatively insensitive measurements, and inappropriate analyses. Nonetheless, the preceding studies have yielded provocative findings and some support for a potentially useful theory.If auditory hallucinations are associated with subvocal activity, this relationship could be helpful in developing an effective intervention technique.” p.619
The paper goes on to compare auditory hallucinations in control (baseline), mouth open, biting tongue, making a fist, raising eyebrows and humming conditions and finds that auditory hallucinations are reduced in the latter humming condition only, consistent with the theory that auditory hallucinations are produced by or related to subvocal speech.
More recent research…
This paper finds that vocal auditory hallucinations are associated with activity in the language producing parts of the brain.
Anthony, D. (2004). The cognitive neuropsychiatry of auditory verbal hallucinations: an overview. Cognitive Neuropsychiatry, 9(1-2), 107-123.
I have asked if the author would post to Research Gate.
This 2013 paper, which is available in pdf on the Net, measured EMG measured micro muscular movements of lips and (as a baseline) the forearm, and found that the lips moved more when auditory hallucinations were present.
Rapin, L., Dohen, M., Polosan, M., Perrier, P., & Lœvenbruck, H. (2013). An EMG study of the lip muscles during covert auditory verbal hallucinations in schizophrenia. Journal of Speech, Language, and Hearing Research.
I have no idea about non vocal auditory illusions such as gunshots, and multiple voices at the same time, and I agree that these phenomena do demonstrate that the theory is weak, or even dumb.
The Russian psychologists, such as Vygotsky and Solovkin saw linguistic thought as developing from children’s babbling that gradually becomes quiet, and thus I presume that subvocal speech is a sort of developmental throwback to the time when thought was spoken. If this theory were correct, it would be reasonable to assume that some or all adults can produce inner speech without sub-vocalisation some of the time, but it would still suggests that inner speech is in essence, or at least in origin, silenced speech. If vocal auditory hallucinations were similar in nature, then humming and other activities that use the vocal cords might help, to an extent.
I am not particularly into humming.
I am interested in what thought is, and what it implies.
Vygotsky claimed that speech and linguistic thought implies a listener, real or imagined. He attempted to demonstrate this by putting young babbling children into a room with other children that do not speak the same language. He found that these children stop babbling on about what they are doing when they realised that no one was listening. He argued therefore that self-talk, and thought, is predicated by the dream of, or accompanied by, an imaginary friend. That was my experience, in my case.
I am reminded of Gollum, in the Hobbit, talking to “my precious,” “the ring”. I guess that was what Tolkien was going on about.