The moment Bhishma's conch sounds, the entire Kaurava army erupts. Hundreds of conches, kettledrums, tabors, drums, and horns blare out all at once. The combined noise is so overwhelming that it fills the entire field of Kurukshetra with a tumultuous roar.
Picture the scene: a vast plain stretching in every direction, hundreds of thousands of soldiers standing in formation, and then suddenly every instrument in the army sounds at the same instant. The word the verse uses is "tumula" (tumultuous), a noise so immense it seems to split the sky.
In ancient warfare, the pre-battle sounding of instruments served two purposes. It raised the morale of one's own soldiers, and it struck fear into the hearts of the enemy. The Kaurava army was making a statement of raw power through sheer volume.