This patient support community is for discussions relating to Speech and Language Disorders. Topics include, but are not limited to: Aphasia,
Apraxia,
Autism and Communication, Developmental Dyspraxia, Motor Speech Disorder, Orofacial Myofunctional Disorders, Paradoxical Vocal Fold Movement (PVFM),
Tongue-Tie (Ankyloglossia), Voice Disorders
I think it has little to do with memory. If it is like what I felt, it is clear to the child that the voice is coming from within the mind (as opposed to it sounding like it is actually being spoken by some invisible person) but it is still a "voice" repeating either the last word or sound/word portion that was said/heard.
I'm glad to say I'm healthy, at age 23, and a college grad. I wouldn't worry if I were you, as I had no mental illness to speak of. I asked one of my UNC college professors about this once and he said that such mental activity is incredibly common and not to worry about. Oh, and for whatever it's worth, I have a BA in psychology.
Maybe take the child in for psychological evaluation if it would relieve you and get an IQ test while you're there. It is actually interesting for me to read about someone else having this problem, as it is a first.