Adam Clarke on Psalms 46:3

Though the waters thereof roar - Waters, in prophetic language, signify people; and, generally, people in a state of political commotion, here signified by the term roar. And by these strong agitations of the people, the mountains - the secular rulers, shake with the swelling thereof - tremble, for fear that these popular tumults should terminate in the subversion of the state. This very people had seen all Asia in a state of war. The Persians had overturned Asia Minor, and destroyed the Babylonian empire: they had seen Babylon itself sacked and entered by the Persians; and Cyrus, its conqueror, had behaved to them as a father and deliverer. While their oppressors were destroyed, themselves were preserved, and permitted to return to their own land.