BG 1.11-13: Therefore, I call upon all the generals of the Kaurava army now to give full support to Grandsire Bheeshma, even as you defend your respective strategic points.
Then, the grand old man of the Kuru dynasty, the glorious patriarch Bheeshma, roared like a lion, and blew his conch shell very loudly, giving joy to Duryodhan.
Thereafter, conches, kettledrums, bugles, trumpets, and horns suddenly blared forth, and their combined sound was overwhelming.
Commentary
Duryodhana urged all the Kaurava generals to make sure they are around Grandsire Bheeshma and give him full support while they defend their own respective positions in the military phalanx. Duryodhana beheld Bheeshma's unassailability as an advantage and wanted to use it as strength and inspiration for his army.
Bheeshma was aware that Duryodhana had no chance of victory as the Supreme Lord Shree Krishna was on the opposite side. However, he understood his grand-nephew's anxiety and to cheer him up he blew his conch shell loudly. In olden days, blowing of the conch shell in the battlefield signaled the start of the war. This also conveyed to Duryodhana that Bheeshma was ready to lead the Kaurava army and he would fight dutifully and spare no pain.
On hearing Bheeshma's call for battle, everyone in the Kaurava army also started playing various instruments eagerly, creating tumultuous sound. Shaṅkhāḥ means conches, paṇav are drums, ānak kettledrums, bhreyah bugles, and go-mukh are blowing horns. All these instruments playing together created a loud pandemonium.