Question:If the New Testament forbids the use of musical instruments in the worship service, why are there musical instruments in the Book of Revelation?
Before answering your question, let's get a bit more precision. The New Testament doesn't forbid the use of instrumental music in worship; that is, there is no direct or indirect statement saying the instruments were not to be used in worship. The problem is that there is no authorization to use instrumental music in the New Testament. Such authorization was given to the Israelites under the law of Moses (see Psalm 150 as an example), but the complete lack of it in the New Testament is noticeable. Since Christians are not allowed to modify the law of Christ (Galatians 1:6-9), we cannot assume approval for something when none is given.
I believe you are referring to one of the following verses from Revelation:
"Now when He had taken the scroll, the four living creatures and the twenty-four elders fell down before the Lamb, each having a harp, and golden bowls full of incense, which are the prayers of the saints" (Revelation 5:8).
"And I heard a voice from heaven, like the voice of many waters, and like the voice of loud thunder. And I heard the sound of harpists playing their harps" (Revelation 14:2).
"And I saw something like a sea of glass mingled with fire, and those who have the victory over the beast, over his image and over his mark and over the number of his name, standing on the sea of glass, having harps of God" (Revelation 15:2).
The book of Revelation is written in symbols. The harps represent something, just as the bowls of incense, the beast, the mark, and the sea of glass all have symbolic meaning. Because a symbol is used, it is a mistake to assume that it physically exists. In other words, just because someone is referred to as a beast, it doesn't mean that there was a physical person with shaggy hair and long sharp teeth.
In the Old Testament, instruments were used to praise God and thus the use of instruments in Revelation is a picture of praise being given to God. You can see this reinforced in the verse following each quotation above.
Even if we assigned a literal meaning to the use of harps in heaven (and why there would be physical instruments in a spiritual realm, I do not know -- notice that these are harps of God, not of earth), it does not then imply authorization for the use of harps in worship here on earth. Notice that incense was being burned in these images as well. Does this mean that the church is thus authorized to burn incense here on earth? Most understand the answer is "no." We cannot reach back into the Old Law to authorize practices under the law of Christ. Neither can we reach forward to another realm to authorize things done during the Christian age on earth. Our covenant was given and confirmed. We cannot alter the terms.
Even if you ignored all of the above, notice that music was not being played to the multitude, but that each person in the multitude is playing. If you want to justify instrumental music by these verses, you need to give everyone in the assembly a harp to play. It would not be an option, but a requirement that everyone play a harp.