Please discuss the relationship between breath, voice and posture.
A meaningful discussion actually requires a book. Richard Miller’s THE STRUCTURE OF SINGING is a wonderful resource for learning about the functions of the voice in detail.
But briefly put, pacing the exhalation of the breath between the vocal cords is what causes the cords to vibrate. The way we pace the breath is dependent on keeping the posture elevated and balanced so that the ribs can stay expanded and stretch the diaphragm low for as long as possible.
Our ability to control and refine the pacing of the breath determines the quality of the vocal vibration. For example, if there is too much air, the voice can sound weak and diffuse, or thick and heavy. If there is too little air the sound can be squeaky or harsh. If the posture is collapsed or hyper-extended the tone is also compromised.