The Lord's message is for all of Israel, also known as the house of Jacob. Jacob was the ancestor of both Israel and Judah.
The witnesses who are told to listen could be the nations mentioned in Amos 3:9, the people of Samaria, or groups of prophets.
the God of Hosts: This title (also in Amos 4:13; 5:14–16, 27; 6:8) shows God as the leader of the heavenly armies. This is the true God of the universe, not a local deity (see also Exodus 15:3).