Why do we sound the shofar?

Because the Holy One, blessed be God, said: Blow me a ram's horn that I may remember to your credit the binding of Isaac, the son of Abraham, and I shall account it to you as a binding of yourselves before Me. The Torah tells us: Abraham look up and behold, he saw a ram caught in the thicket by its horns [Genesis 22:13]. This teaches us that the Holy One, blessed be God, showed our ancestor Abraham the ram tearing himself free from one thicket and becoming entangled in another.