the mountains - namely, which separate Media and Assyria, and on one of which the banner to rally the hosts is supposed to be reared.
tumultuous noise - The Babylonians are vividly depicted as hearing some unwonted sound like the din of a host; they try to distinguish the sounds, but can only perceive a tumultuous noise.
nations - Medes, Persians, and Armenians composed Cyrus’ army.