Making the shift from passive awareness to active listening of sound is what makes singing possible, despite protestations by some vocal pedagogues that the singer should not listen to the voice, but should feel it instead. They are half-right insofar that listening, in addition to involving audition of bone and air conduction, also involves vestibular information. This is the third route of awareness of which many speak, yet few know that its true origin lies within the ear, which Tomatis addresses in his work.
If you haven't yet studied Tomatis' The Ear and the Voice, I encourage you to obtain a copy and make your way to pages 86 and 87 (copyright infringement prevents me from quoting the text in toto), where you will find an excellent exercise for acquiring an open ear—what Tomatis describes as Listening Posture, and what I believe brings about the correct opening of the mouth described by Giovanni Battista Mancini in Practical Reflections on the Figurative Art of Singing (1777) as well as the "singing posture" described in Hidden in Plain Sight: The Hermann Klein Phono-Vocal Method based upon the Famous School of Manuel Garcia (2013).
It is the first and most important step for the student of voice, while also being the most difficult for those lacking the necessary audio-vocal control: one which facilitates all styles and timbres of the voice, and enables the "imposto" which is part and parcel of the teaching of the Garcías.
How to practice Tomatis' exercise from a practical standpoint? My humble suggestion is that the student understand his instruction as being initiated before inhalation, sustained during exhalation, as well as involving the elongation of the spine and the opening of the ribs—for therein lies the full expression of the open ear.