If we define prophecy as in the last post, then it is a gift comparable to, evangelist, preacher, or teacher. Are these considered to be infallible? No. Are they considered to be empowered by the Holy Spirit? Yes. So let's remove the idea of infallibilty from the discussion and answer the question at hand. Who speaks in prophecy? The prophet. He speaks on the behalf of God. He gives the message that has been given to him by God.
Is this prophecy separate from Scripture? No. Just as preaching and teaching need to be examined inthe light of Scripture prophecy is to be examined by Scripture. If there is anything contained in the prophecy that does not align with Scripture, throw it away. Scripture is always the final authority.
But it is also God who is speaking in prophecy. And He is speaking in a way that is different than in normal preaching. He speaks to us concerning our sins. Now we know that in the Bible there is not an exhaustive list of does and don'ts. But in Scripture are the principles that teach us right from wrong, even when a specific sin is not mentioned. A quick illustration of this is drugs. Drugs are not mentioned in Scripture, but we know that drugs are wrong. How do we know? Because we have revealed for us in Scripture that our bodies are the dwelling place of the Holy Spirit, therefore we should not give our bodies drugs that will destroy the body. This is an over-simplification I'm sure, but the idea is that this is revealed to us by the Spirit, not separate from Scripture, but together with Scripture. The letters do not spell those words out, but the Spirit shows us our sin through the words that are written.
Can these intimations be fallible? Yes. Many times it is not the Spirit who speaks something, but it comes from our own imaginations. The burden of the prophet in these matters is on comparing what he would say with Scripture before he says it.
Then there are another group of prophecies that come spontaneously upon the prophet. these are often revealed through meditation on the Word. These are words that many times the prophet is not sure that are needed, but is impressed by the Spirit to speak them anyway. Read Phil Johnson's post. Phil is a cessationist, as is Spurgeon, but those quotes illustrate what I would call prophecy.
Can prophets tell the future? This is the question that comes to mind first when we pseak of prophecy, yet it is not the prophet's primary focus. I would answer the question, yes. It is possible for the Holy Spirit to reveal the future to a prophet. Whether or not this happens often is another question. I would say that it is very rare that the future is revealed through a prophet. But it has happened I believe. One notable prophecy was spoken by John Huss as he was about to be burned at the stake.
John Huss died a martyr's death for preaching the Gospel. At some point inthe proceedings, before he was burned, he said, "You are now going to burn a goose, but in a century you will have a swan which you can neither roast nor boil."
John Piper records for us in the introduction to The Legacy Of Sovereign Joy, these words of Martin Luther,
"John Hus prophesied of me when he wrote from his prison in Bohemia: They will now roast a goose (for Hus means goose), but after a hundred years they will hear a swan sing: him they will have to tolerate. And so it shall continue if it please God." Piper, John. The Legacy Of Sovereign Joy, (Wheaton: Crossway Books, 2000) 10-11.
Prophecy is not contrary to Scripture. It does not open a prospect for further revelation. Let's not balk at this gift that God has given the church.