439. The cause of dumbness. 

We are aware that deficiency in hearing leads to deafness, and issues in eyesight leads to visual impairment.

But issues in the mouth are not attributed as a reason for dumbness in humans.

When there is deficiency in hearing in a person, the words spoken by others do not get registered in the hearts of the listener. 

In verse 2:18 of the Quran the Almighty while talking about hypocrites calls them, deaf, blind and dumb.

The reason for all these three deficiencies is seen in verse 2:7 of the Quran.

This verse (2:7) says their hearts and ears are sealed and their eyes are blinded.

It implies they are blind because their sight is blocked, and because their ears are sealed, they are deaf.

These are facts everyone is aware of for ages. 

But when talking about them being dumb, the Quran says their hearts are sealed, and not their mouths.

The fact, deafness in a person is the result of the sealing of his heart was discovered several years after Prophet Muhammad.

An illiterate like Prophet Muhammad cannot have declared the same on his own, with the help of knowledge gained from the society that existed at his time. Hence this is yet another proof the Quran is the word of God.