Revelation 14:2-5

2  And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and the voice which I heard was as the voice of harpers harping with their harps:

3  and they sing as it were a new song before the throne, and before the four living creatures and the elders: and no man could learn the song save the hundred and forty and four thousand, even they that had been purchased out of the earth.

4  These are they that were not defiled with women; for they are virgins. These are they that follow the Lamb whithersoever he goeth. These were purchased from among men, to be the firstfruits unto God and unto the Lamb.

5  And in their mouth was found no lie: they are without blemish.

John hears the voice of Jesus as the sound of “many waters”. The voice of great thunder is probably the Father’s voice pronouncing judgment on the earth. The thunder could contain the seven thunders that we were told about in chapter ten, but what the thunders contain is sealed until the appointed time. John also hears heavenly background music for the song that the 144,000 sing before all the host of heaven. Only the 144,000 purchased Jewish brethren who have been redeemed by the blood of Christ can learn this new song. These men have followed in the footsteps of Jesus for 3 ½ years of ministry on earth. They have proclaimed the gospel of the kingdom as a witness to the whole world.  These are called first fruits unto God and the Lamb,  so before these men arrive in the Holy City the rewards could not have been given to the Church and the marriage could not have taken place in heaven. 

These men have never married and God sees them as being spiritually pure. These 144,000 are the best of the crop and they are hand picked before the general harvest occurs.  These brethren go with Jesus wherever He goes, so they are the same ones standing with Jesus when He gathers the nations before Him at the sheep and goat judgment described in Matthew chapter 25.  How the survivors of the tribulation treated these 144,000 Jewish brethren will determine whether they will go into the kingdom on earth or be gathered to be crushed in the great winepress of God.