This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

v. To articulate or engage in articulation by moving the lips or other speech organs without making audible sounds, as in reading to oneself.

v. To form words or statements in thought and express them inwardly without uttering them aloud.

v. articulate without making audible sounds

"Most readers, and precisely because they subvocalize in the production of a text, are likely to be taken momentarily aback by this thumbnail sketch of"

articulate    enunciate    say    sound out    pronounce    enounce   

subvocalization