"All Scripture is God-breathed" (2 Tim. 3:16).
God has spoken. He has spoken in human language. He has spoken in words. When we open the Bible, we have God's word written. If you want to hear the voice of God, and if you want to be sure it is the voice of God, turn to Matthew. Or Deuteronomy. Or Philemon. Or Malachi. When we read the Bible, we hear the voice of God. It is God's word written.
As I said the other day, sermons must always be submitted to the discernment of the church. This is Paul's teaching (1 Cor. 14). The issue is not the degree of authority but discernment of what is authoritative. Thus, prophecy as revelatory speech directly inspired by God must always be carefully distinguished from ordinary preaching of the Scriptures. Any purported preaching of divine truth must meet the biblical pattern: 1) It must be judged by the community as harmonizing with canonical revelation; 2) it must be edifying to the church; and 3) it must be done in an orderly manner in accordance with Paul's instructions to the Corinthians.
In the end, all preaching must be both authoritative and tentative (distinguishing between the infallible word and its fallible interpreters).