Listen! Hear Him! Believe what He says!
The author of Hebrews has presented us with the facts that Christ is greater than angels, greater than Moses and greater than Aaron; that the New Covenant is superior in every way to the Old Covenant. He addresses this presentation to Christians who stand, not before a physical mountain in the Sinai, but a spiritual Mount Zion in heaven. Nevertheless, we still have the potential to refuse to hear, even as the Israelites who had just come out of Egypt did not hear. Now, they knew—they knew—that it was the voice of God that they heard, and they refused to hear because they believed they could not endure what He commanded!
Do we see the parallel?
It is possible for Christians to cherish their own will—which they know to be diametrically opposed to the will and purpose of God—and to stick to their own desires, thus stifling the voice of the Almighty God Himself! And thus, we can wrench ourselves away from the voice because we feel uncomfortable going against our resolve.