Words related to singing, in the King James, appear eight times within Revelation's songs (Rev. 5:9, 14:3, 15:3). This is significant, as eight symbolizes a new beginning, order or creation. The praises in this prophetic book all rejoice at events that mark a new beginning for humans made possible by God's perfect love.
Praising God's Plan
The first song of Revelation is recorded in chapter 5. It will be triggered when, in the near future, Jesus takes a prophetic scroll from God's right hand (Rev. 5:6 - 7). The loosening of the scroll's seals will kick off events that will ultimately lead to Christ's Second Coming and the establishment of God's Kingdom on earth (chapter 19).
And they (four living creatures and 24 elders) sang a new song: "You are worthy to take the scroll and to open its seals, because you were slain, and with your blood you purchased men for God from every tribe and language and people and nation. You have made them to be a kingdom and priests to serve our God, and they will reign on the earth" (Revelation 5:9 - 10, NIV).
Jesus' sacrifice makes possible the New Covenant (Matthew 26:27 - 28) and, when compared to the Old Covenant, its far superior promises (Hebrews 8:6). The praise in Revelation 5 is new because it is based on this new agreement between God and man.
The second part of this prophetic song focuses on all the righteous dead brought back to life at Jesus' return (1Corinthians 15:23, 50 - 52, 1Thessalonians 4:16 - 17). Each of them will be granted eternal life and allowed entrance into God's spiritual family. They will also be honored with becoming a royal priesthood that will, under Christ, reign over the earth for 1,000 years (2Timothy 2:12, 1Peter 2:9, Revelation 1:6, 5:10, 20:6).
Salvation of 144,000
And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth (Revelation 14:3, KJV).
Those who will sing the prophetic song of Revelation 14 will be a special group of 144,000 Israelites (not just Jews) God chooses to convert during the Great Tribulation period (Rev. 7:1 - 8). Their extraordinary experiences at overcoming the Beast and False Prophet (Rev. 7:13 - 14, 13:1 - 17), as Israelites, will lead to their unique praise mentioned in this chapter.
Overcoming Evil
And they were singing the song of Moses, the servant of God, and the song of the Lamb, saying, "Great and awesome are Your works, Lord God Almighty; righteous and true are Your ways, King of the saints. Who shall not fear You, O Lord, and glorify Your name?
For You only are holy; and all the nations shall come and worship before You, for Your judgements have been revealed" (Revelation 15:2 - 4, HBFV).
Revelation 15's prophetic song will be generated by those who will live during the Great Tribulation period (Rev. 15:2). They are those not only called of God but also are obedient to His commandments (Rev. 12:17, 14:12, 22:14) during the worst time in human history (Matthew 24:21).
The song of Moses (Exodus 15:1 - 18) is a reference to the praise generated after the Egyptian army's complete destruction in the Red Sea (14:15 - 31). This miraculous saving of the Israelites, as they were fleeing Pharaoh, guaranteed their freedom from slavery.
Those who spiritually overcome the forces of evil during the Great Tribulation will be rewarded, like the ancient Israelites, with a complete victory. They will be permanently saved from their enemy, the devil, when they stand before God on a sea of glass.
Conclusion
The prophetic songs of Revelation look forward to a time when some of the greatest parts of God's salvation plan will finally be fulfilled. Its second and third songs are offered by those who are part of the first resurrection praised in Revelation 5. All those raised from the dead will rejoice in receiving their spiritual inheritance promised by God (Ephesians 1:11 - 18, Colossians 3:24, Hebrews 9:15).