en_tn_condensed/isa/13/04.md

The noise of a multitude in the mountains, as of many people

The words "there is" are understood. The phrases "multitude" and "many people" mean the same thing. AT: "There is the noise of many people in the mountains" or "There is the noise of a great crowd of people in the mountains" (See: rc://en/ta/man/translate/figs-ellipsis)

The noise of a tumult of the kingdoms like many nations gathered together

The words "there is" are understood. The words "kingdoms" and "nations" here refer to the same thing. AT: "There is the noisy commotion of many kingdoms gathered together" (See: rc://en/ta/man/translate/figs-ellipsis)

is mustering

"is gathering"