The Diaphragm controls how the breath and voice flows.
 

Under stress and anxiety, the Hypothalamus of our brain triggers and activates the pituitary gland to release ACTH. ACTH hormone stimulates the Adrenal glands and this initiates flow of Adrenalin (a hormone) to blood.

 

Enhanced levels of Adrenalin, can trigger the fight-flight mechanism; shallow chest breathing, increased heart rate and can make a person feel suffocated in the 'gut' (the diaphragm) resulting in voice coming out weak.

 

Controlling the breath for public purposes is similar to performing breathing exercises and speech is nothing but controlled outward breath.