Wiktionary
n. 1 (context not countable English) The formation in thought of words or statements, which are expressed inwardly but not uttered aloud. 2 (context countable English) subvocal words or statements so formed.
Wikipedia
Subvocalization, or silent speech, is the internal speech typically made when reading; it provides the sound of the word as it is read. This is a natural process when reading and it helps the mind to access meanings to comprehend and remember what is read, potentially reducing cognitive load. This inner speech is characterized by minute movements in the larynx and other muscles involved in the articulation of speech. Most of these movements are undetectable (without the aid of machines) by the person who is reading. It is one of the components of Baddeley and Hitch's phonological loop proposal which accounts for the storage of these types of information into short-term memory.
Usage examples of "subvocalization".
He spread garnetberry jam on his roll and switched to silent subvocalization.
I pick up your subvocalizations via your throat muscles, translate them, and broadcast my words back to you.
He was somewhat distracted by the verbal murmuring sounds coming from Aari and Acorna, subvocalizations that were almost purrs.