Gill, in his commentary, gives us a useful summary of this chapter.
‘This chapter relates the appointment of the sons of Asaph, Heman, and Jeduthun, to be singers in the temple, 1 Chronicles 25:1 the distribution of them by lot into twenty-four classes, twelve in each class, 1 Chronicles 25:8.’
David, along with the commanders of the army, 1 Chronicles 23:2 / 1 Chronicles 24:6, set apart the sons of Asaph, Zakkur, Joseph, Nethaniah, and Asarelah, under the supervision of Asaph, who himself prophesied, or performed the sacred services, under the supervision of the king. There are 12 Psalms are attributed to Asaph, Psalm 50 and Psalm 73-83.
To prophesy with a harp simply means that one used the music that was produced with the harp to minister to the ears of the hearers. Their purpose in the temple was to produce a tranquil environment for the people. The Good News Bible says, ‘they were to proclaim God’s messages accompanied by the music of harps and cymbals’.
Matthew Henry, in his commentary, says the following.
‘David put those in order who were appointed to be singers and musicians in the temple. To prophesy, in this place, means praising God with great earnestness and devout affections, under the influences of the Holy Spirit. In raising these affections, poetry and music were employed. If the Spirit of God do not put life and fervour into our devotions, they will, however, ordered, be a lifeless, worthless form.’
Barnes, in his commentary, says the following, concerning the horn.
‘Some take this literally and consider that Heman and his sons played on the born in the musical services but there is no other evidence that the horn was so employed. Perhaps the most probable explanation is that it has been transferred from the next clause, where, as here, it followed the word ‘God’, with the sense that ‘God, to exalt Heman’s horn, or increase his dignity, gave him fourteen sons and three daughters’.’
Each son of Asaph, Jeduthun, and Heman, was at the head of a band of twelve skilled musicians, consisting partly of his own sons, partly of Levites belonging to other families, 1 Chronicles 25:9-31.
The 24 band leaders, together with their bands, formed a body of 288 persons. Besides these, we hear of there being above 3,700 singers, who were probably divided, like the trained musicians, into 24 courses, which must have contained about 155 each, 1 Chronicles 23:5.
Notice David didn’t make a choice in terms of who was the most talented musician for worship, he let God do the choosing.
Barnes, in his commentary, says the following, concerning the lot.
‘The lot was not applied indiscriminately to all the 24 courses but was only used to settle which course of Asaph, which of Jeduthun, and which of Heman, should on each occasion be taken. Asaph was given precedence over his brethren, and his four courses were assigned the first, and then each alternate place. Jeduthun took rank next, and received alternate places, first with Asaph, and then with Heman, until his courses were exhausted. After this, all the later places fell necessarily to Heman, whose courses continue without interruption from the 15th.’
It’s important to note that David was the one who invented musical instruments to be used in worship, it was never a part of God’s plan. In a sense, this was the beginning of what later in Israel’s apostasy became a thing that was an irritating sound before God, Amos 6:5.
When they went into apostasy, Amos preached the words of God that they take away the noise of their songs and stringed instruments, Amos 5:23.
"If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness."