Adam Clarke on Job 3:18

The prisoners rest together - Those who were slaves, feeling all the troubles, and scarcely tasting any of the pleasures of life, are quiet in the grave together; and the voice of the oppressor, the hard, unrelenting task-master, which was more terrible than death, is heard no more. They are free from his exactions, and his mouth is silent in the dust. This may be a reference to the Egyptian bondage. The children of Israel cried by reason of their oppressors or task-masters.