This chapter opens with the concept of holiness. David called the chief leaders to set aside Asaph (Psalm 50, 73-83), Heman (Psalm 88), and Jeduthun (Psalm 39, 62, 77) for service. This is the concept of holiness. We have been chosen by God for a particular service by a particular gifting. When we follow His purpose for our lives, we are living in accordance with His will and holiness. When we choose to go against His purpose, we are living unholy lives. This does not negate the fact though that we are still holy, set apart for Him.
These men were set apart to prophesy with music. This can be seen as telling forth in God’s Word with a meter in such a way that God uses it to convict and convince those who hear. It does not necessarily mean speaking a new word from God, particularly in the church of today (Hebrews 1.1-4). The meaning of the term “prophesy” here seems to mean “to proclaim” or “to praise.” The term, therefore, would cover expressing what God’s will is to those who listen, not a new word, but an expression of the known will. Paul reminds us of this in 1 Corinthians 14.3.
Some see verses 3-4 as stating that God speaks prophetically to His people, at times, without the human voice. I have not researched this enough to write on it now; however, it does intrigue me. If you desire more discussion on this, let me know.