Are there prophets today? Well, no. And yes. If you mean, "Are there prophets like Israel had in the Old Testament?", then no. If you mean, "Do people speak with prophetic voices today?", yes.
The phenomena of Old Testament prophecy can be traced historically as well as culturally. Setting aside the influences from surrounding cultures (and they were considerable), it is helpful to note the time frame in which Old Testament prophecy flourished. Even though Moses is referred to anachronistically as the paradigm of a prophet (Deut 34:10), Old Testament prophecy arose with Samuel in the eleventh century BC, and all but died out by the sixth century BC in the post-exilic era. There were a few prophets after the exile, but beginning with Ezekiel most were also priests and served a slightly different role in Israel (Haggai, Zechariah). Malachi, around 450 BC, is considered the last of the Old Testament prophets (see Israelite Prophets Date Chart).
It is no coincidence that the beginning of Old Testament prophecy coincided with the rise of monarchy, and that the decline of prophetic activity also coincided with the demise of the monarchy. The first prophet, Samuel, was a kingmaker. And the ministry of the last pre-exilic prophet, Jeremiah, was marked by conflict with Israel's King Jehoiakim.
In fact, prophets were often in conflict with kings. Nathan dared confront David with his murderous sin. Amos was asked to leave the Northern Kingdom because he dared speak against the king (Jereboam II) in one of his own royal temples. Both Micah and Isaiah fiercely attacked the pro-Assyrian political intrigue and religious syncretism of Ahaz. And even John the Baptist felt the responsibility to challenge Herod, with deadly consequences.
In looking at Old testament prophets more closely (beyond the narrow category of prediction), it is clear that their message was most often calling the people back to proper worship of God. But much of that task was done in the context of the community, the nation, of Israel. That meant that much of the criticism of the prophets was leveled at religious leaders (which included what we would call "political" leaders) for their failure to be spiritual leaders. It was also aimed at the powerful, most often also the religious leaders, who used their power and influence for selfish or sinful purposes. The prophets were a balance to the unrestrained power of the monarchy and the aristocracy (cf. 1 Sam 8:11-17).
All this says that OT prophets served two complementary roles in Israel. They spoke for God to the people, calling the people to respond faithfully to the God who had revealed Himself in their history. But they also spoke for the weak, the oppressed, the disenfranchised, those who had little voice in shaping their own lives or their own future. Even in that role, they were still speaking for God, because their tradition remembered that once they were slaves in Egypt with no voice in their own future until God entered history and delivered them. Such oppression of the helpless by the powerful was understood to be a violation of the most fundamental part of God's revelation of Himself, that He is the kind of God who hears the cries of oppressed slaves and responds with grace and deliverance.
So the prophets stood as a counter voice to those who would allow the allure of power, ambition, and self-serving self-righteousness to blind them to the things of God: doing justice, loving mercy, and walking humbly with God. They were, in the best sense of the term, "counter-culture" Israelites. As Walt Brueggemann writes (The Prophetic Imagination), they called the people to live in an alternate reality not governed by the rules of power and success. They called them rather to live out Torah as a faithful response to God. They called the people to abandon the status quo shaped by those who benefited from it the most, to embrace a new future shaped, empowered, and energized by God.
So, as Abraham Heschel writes, the prophets always sang one octave too high. They were empowered by a vision of how things could be, a future in which the people and their leaders would live out their calling to be the people of God as a channel of blessing to the world. And the prophets had the courage to call into question any preoccupation with the status quo on any level that interfered with that future. As a result, they were often in trouble with those who stood to lose the most if the status quo were changed and that "could be" future became a reality.
In another context, this same idea was embodied in Jesus as he talked about the first becoming last and the last becoming first, and as spoke and lived the idea that true leaders were servants who washed others' feet. In fact, this "alternative reality" is the heart of the "Kingdom of God" in the New Testament.
There are other issues related to what makes an Old Testament prophet, such as their calling by God (see The Prophetic "Call" Narrative: Commissioning into Service). However, because of the cultural aspects of the Old Testament, there are no more prophets in the strict sense of the word. That was a uniquely historical phenomena that cannot be duplicated now because history has changed. But the prophetic concern with faithful response to God, especially as it plays out in issues of power, authority, and control, becomes the basis for talking about a modern prophetic voice, one who speaks prophetically.
The issues that called forth the prophetic messages of the Old Testament are very contemporary issues. There are still Davids around, who, even though they may be good leaders and people of God at times, sometimes use their power in horribly destructive ways to achieve their own ambitious or selfish ends. There are still people like Ahaz who are so blind to the things of God that they are willing to build altars to whatever gods they think are the most powerful in order to extend their ambition and control.
There are still Jeroboams who are more concerned with wealth, success, and empires than they are with the suffering their ambition causes for others. There are still Herods caught between their own delusions of grandeur and even more powerful political forces, who are willing to sacrifice whomever is necessary to secure their own comfort and survival. There are still Pharisees who are so sure that their way is God's way that they are easily willing to crucify anyone who poses a threat to the status quo of their version of the truth.
And so, I think, there needs to be people today, Christians today, who will dare to stand and speak the truth in love, who will dare to stand before the king and say, "You are the man!" Here it is easy, especially in more conservative traditions of Christianity to assent rather quickly to the idea that we should "take a stand" against all the sin in the world. We easily assume that the enemy is external, a force of evil that threatens to overcome all of us righteous people. There is a truth there. But it is not the whole truth. The voice that decries sin in the world is not a prophetic voice.
A truly prophetic voice is one who has the courage, perhaps even in some sense the calling of God, to look around at the community of faith in its status quo and say, "Not everyone who says 'Lord, Lord' will enter the kingdom of heaven." A prophetic voice is one who calls God's people to return to their calling as His people. A prophetic voice is one that will not settle for the status quo, not for the sake of stability, or security, or comfort, or even for the sake of conserving the tradition. A truly prophetic voice is a radical voice, a liberal voice that calls for change, even if that change is a return to a vital tradition long obscured by false piety and self-righteousness.
A prophetic voice will not gloss over injustice or oppression, will not be silent in the face of bigotry or prejudice or false pride, and will not compromise faithfulness for practical ends no matter how noble those ends may be in themselves. A truly prophetic voice is one that will sweep away all the trappings of religion and simply ask, "What does God require?", and answer simply, "do justice, love mercy, walk humbly with God." Or simply "love God, love others." A prophetic voice is one that will settle for nothing less than holiness of heart and life as the result of faithful obedience to the voice of God. In a real sense, a prophetic voice even today is the voice of God.
In our culture, and with our history, it is easy to claim the role of a prophet. But it is also easy to claim that role as its own form of ambition and power. It is perhaps too easy to claim to speak prophetically, but to do so with arrogance, anger, and bombast as David Koresh well demonstrated. A truly prophetic voice speaks in love, not anger, even when it cries "Woe to you hypocrites!" Jeremiah spent 40 years of his prophetic ministry telling the people, sometimes in the harshest and coarsest language possible, that they were sinners and were going to die. But he took no joy in that message. At the same time that he stood firm as the lone prophetic voice against the tyranny, injustice, and idolatry of God's people, he was weeping. He told them they were going to die for their sins, but he did so with tears in his eyes (for example, Jer 8:22-9:1)!
And Jesus on more than one occasion soundly denounced religious folk who could not envision anything beyond their own little world of truth. And yet, he stood overlooking the city of Jerusalem and wept over it, knowing that the very people over whom he was weeping had murder in their hearts.
Do we need prophetic voices today? Perhaps. A few. Not everyone can be a prophetic voice. It's costly. Prophets often get thrown into cisterns and tried for treason. Or beheaded. Or crucified. Or shot. It's just not a very popular position, so it takes a certain kind of person to be a prophetic voice.
Prophetic voices just aren't "team players." They just don't understand that you can't say certain things or make certain people mad. They just don't understand that bills have to be paid and buildings have to be built. They just don't understand how the system works and how things get done.
Prophetic voices are often on the fringe of the crowds who flock to hear the latest "how to" speaker. They just don't fit in with polite conversation about the latest triviality, because they want to talk about much too serious topics. They are socially insensitive because they continually want to bring up topics no one else wants to talk about, and are often politely told that they should find a more appropriate forum, perhaps at that "other" church that really needs to hear it. Prophetic voices often see things that others do not see, and so are often an embarrassment to their "family," who sometimes accuse them of being just a little mixed up.
How does one become a prophetic voice? I don't know. I think there are a variety of factors, but no one thing that I can name. Sometimes life experiences can open our eyes to the injustice and prejudice around us. Sometimes the passion of others provides a model for that "alternate reality." Sometimes the written word of God may call us to see beyond where we are. Sometimes the Holy Spirit opens our eyes to a new vision of His work in the world. Sometimes it is a personality trait that is sensitive to certain things. Sometimes a deep commitment to the things of God leads to a spiritual growth that opens up new vistas. Sometimes, perhaps, God simply speaks.
I don't know how to become a prophetic voice. But I wouldn't recommend it anyway. There's not much future in it. At least, not now.