This is a great question. 2 Kings 19:31-36 concerns the three main players: Hezekiah, Isaiah, and Sennacherib. Here is the context. During King Hezekiah's rule over Judah, Sennacherib, the Assyrian King threatens Judah. This was a real problem for Judah, because the kingdom of Assyria was the most powerful in the area and doom was certain if Assyria went through with the attack of Judah.
In light of this circumstance, Hezekiah prayed to God and asked for help. He knew that deliverance could only be found in God. On account of the sincerity and piety of Hezekiah, God answered his prayers. Isaiah the prophet goes to Hezekiah and lets him know that God will save them. 2 Kings 19:31-24 contains the words of encouragement from God through Isaiah.
As the story unfolds, an angel of death visits the army of Sennacherib and kills 185,000 Assyrians according to 2 Kings 19:35.
As for the blowing of the horn, the only clear reference to this is found in Joshua 6 and this symbolizes that God has given the battle to his people. This is where we read of the fall of Jericho.