1 Chronicles 25:1-3 David and the chiefs of the service also set apart for the service the sons of Asaph, and of Heman, and of Jeduthun, who prophesied with lyres, with harps, and with cymbals. The list of those who did the work and of their duties was: Of the sons of Asaph: Zaccur, Joseph, Nethaniah, and Asharelah, sons of Asaph, under the direction of Asaph, who prophesied under the direction of the king. Of Jeduthun, the sons of Jeduthun: Gedaliah, Zeri, Jeshaiah, Shimei, Hashabiah, and Mattithiah, six, under the direction of their father Jeduthun, who prophesied with the lyre in thanksgiving and praise to the Lord.
Here we are told that the sons of Asaph, Heman, Jeduthun prophesied with their musical instruments. If we understand prophecy in the OT as mainly God speaking through a prophet, then prophesying with musical instruments would mean God using not the words (spoken or written) but music to communicate his thoughts and feelings to God's people. Paul in 1 Corinthians 14:3 defines the aims of prophecy as God speaking "to people for their upbuilding and encouragement and consolation." So, similarly God can "speak" through music played under the inspiration of the Spirit to build up (edify), to encourage and to console (comfort). So, music is not just for the purpose of "accompanying" the human voice, but is and by itself a channel of divine thoughts and emotions to edify, encourage and comfort the listeners.
How does it work? Scripture tells us that God is Spirit and He communicates with us via our human spirits, not the human minds: in other words, God's thoughts are first "apprehended" before they are translated into words or music: "these things God has revealed to us through the Spirit. For the Spirit searches everything, even the depths of God. For who knows a person's thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God. Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God. And we impart this in words not taught by human wisdom but taught by the Spirit, interpreting spiritual truths to those who are spiritual." (1 Corinthians 2:10-13),
Implied in this is that the Spirit of God who alone knows the depths of God communicates them to us who are "spiritual" (spiritually alive), so that the mind may then express them in words spoken or written. So, in the same manner, the Spirit speaks first to our spirits, and we can also express these "thoughts" of God in musical expressions. When our music is inspired by the Spirit, it becomes prophetic, not just human music.
May the Lord open the hearts of all who lead worship to be prophetic in their ministry!
Father, inspire us by Your Spirit that we might use music to edify, to encourage and to comfort those who come to worship You. Amen.