subvocal

/S AH0 B V AH0 K AH0 L/
adjective
  1. 1

    Of or pertaining words or statements formed in thought and expressed inwardly but not, or not yet, uttered aloud.

  2. 2

    Of or pertaining to imperfectly articulated speech that is inaudible or barely audible; subtonic.

Source: https://en.wiktionary.org/wiki/subvocal