The title says The title of Psalm 81 says it is by Asaph, for the Chief Musician, on an instrument of Gath. It moves from joyful worship to sober warning. The opening calls the community to the festival with song, tambourine, lyre, harp and trumpet. The feast is a statute for Israel, connected to God going out over the land of Egypt.
From verse 6 God speaks. He recalls redemption: I removed his shoulder from the burden. His hands were freed from the basket, a picture of forced labor. He recalls testing: I tested you at the waters of Meribah. Then he restates the heart of the covenant: I am Yahweh, your God, who brought you up out of the land of Egypt. Open your mouth wide, and I will fill it. God's generosity is unlimited, but it must be received.
The tragedy is refusal. My people didn't listen to my voice. Israel desired none of me. So I let them go after the stubbornness of their hearts. God's judgment here is to give them what they chose. Yet he still pleads: Oh that my people would listen to me! Obedience would bring victory and abundance, the finest of the wheat and honey out of the rock. Worship that does not listen misses the feast.