The phrase "lift up their voices" is an idiom that means to speak loudly. AT: "They will sing and shout about the majesty of Yahweh" (See: [[rc://en/ta/man/translate/figs-idiom]])
Here "the sea" refers to the Mediterranean Sea which is west of Israel. AT: "and those in the west towards the sea will shout with joy" (See: [[rc://en/ta/man/translate/figs-explicit]])