Mouth moves aping sound
Single simple words suffice
Nothing more to say
n.b. Selective mutism is an intriguing behaviour. Despite having the facility to speak there are some people who simply do not.
“Why not?” is a question that remains in the realm of speculation until the person is able to engage in conversation independently.
At the other end of the scale there are those who choose not to listen. This is a different problem; one that involves choices.