First Corinthians 14 is one of the most important chapters in the Bible concerning divine worship. In that chapter, the apostle Paul provides teaching about musical instruments that reveals a truth that vitally pertains to the use of an instrumental number in worship.
1Co 14:7 And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped?
8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
These verses explain that a musical instrument, such as a pipe (flute) or a harp, is a nonliving entity that produces sound. For the hearer to know what is played on that instrument, it must produce a distinction in the sounds that it makes.
Paul supports this teaching by giving the example of what was true about the use of a trumpet to alert people to prepare themselves for a battle. Through a question that demands a negative answer, he asserts that no one will prepare himself for a battle if a trumpet gives an indistinct sound.
From this teaching, we understand that a proper use of an instrument in an instrumental number in worship requires that the hearers know what is being played on it by paying attention to the distinct sounds that it makes. Unless, therefore, one is certain that every hearer of an instrumental number in a service will know exactly what the song is that is being played and what all the words of the song are, the words of the song must be provided to the hearers in some manner so that they know what is being played on the instrument.