Adam Clarke on Luke 1:20

Thou shalt be dumb - Σιωπων silent; this translation is literal; the angel immediately explains it, thou shalt not be able to speak. Dumbness ordinarily proceeds from a natural imperfection or debility of the organs of speech; in this case there was no natural weakness or unfitness in those organs; but, for his rash and unbelieving speech, silence is imposed upon him by the Lord, and he shall not be able to break it, till the power that has silenced him gives him again the permission to speak! Let those who are intemperate in the use of their tongues behold here the severity and mercy of the Lord; nine months' silence for one intemperate speech! Many, by giving way to the language of unbelief, have lost the language of praise and thanksgiving for months, if not years!