Albert Barnes Commentary Job 29:9

"The princes refrained from talking, And laid their hand on their mouth;" — Job 29:9 (ASV)

The princes refrained talking – As a mark of respect, or in awe of his presence.

And laid their hand on their mouth – To lay the finger or the hand on the mouth is everywhere an action expressive of silence or respect (Job 21:5). “In one of the subterranean vaults of Egypt, where the mummies lie buried, they found in the coffin an embalmed body of a woman, before which was placed a figure of wood, representing a youth on his knees, laying a finger on his mouth, and holding in his other hand a sort of chafing dish, which was placed on his head, and in which, without doubt, had been some perfumes.” Maillet.