7 While you are rejoicing, and delighted with the ductile trumpets, and the voice of the horn, what follows? “Let the sea be stirred up, and the fullness thereof”. Brethren, when the Apostles, like ductile trumpets and horns, were preaching the truth, the sea was stirred up, its waves arose, tempests increased, persecutions of the Church took place. Whence has the sea been stirred up? When a joyful noise was made, when Psalms of thanksgiving were being sung before God: the ears of God were pleased, the waves of the sea were raised.
“Let the sea be stirred up, and the fullness thereof: the round world, and all that dwell therein.” Let the sea be stirred up in its persecutions. “Let the floods clap their hands together”. Let the sea be aroused, and the floods clap their hands together; persecutions arise, and the saints rejoice in God. Whence shall the floods clap their hands? What is to clap their hands? To rejoice in works. To clap hands, is to rejoice; hands, mean works. What floods? Those whom God has made floods, by giving them that Water, the Holy Spirit. “If any man thirst,” says He, “let him come unto Me, and drink. He that believes in Me, out of his bosom shall flow rivers of living water.” These rivers clapped their hands, these rivers rejoiced in works, and blessed God. “The hills shall be joyful together.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)