John Gill Commentary 2 Chronicles 7:6

"And the priests stood, according to their offices; the Levites also with instruments of music of Jehovah, which David the king had made to give thanks unto Jehovah, (for his lovingkindness [endureth] for ever,) when David praised by their ministry: and the priests sounded trumpets before them; and all Israel stood." — 2 Chronicles 7:6 (ASV)

And the priests waited on their offices
Performed them, some in offering sacrifices, others in blowing trumpets, as it may be explained from the latter part of the verse:

the Levites also with instruments of music of the Lord, which David the
king had made to praise the Lord ;
under a divine direction, on which the Levites played to the songs of praise offered to the Lord, and by which they made music sacred to him:

because his mercy endureth for ever ;
which, as it was the close of their songs of praise, was the cause of them:

when David praised by their ministry ;
the songs sung being composed by him, and the instruments they played upon being of his invention, and used by his order:

and the priests sounded trumpets before them ;
or rather over against them, that is, over against the Levites, as they were singing and playing on the instruments of music:

and all Israel stood ;
while this sacred and delightful service was performing, they both stood up, and stood by the priests and Levites, and joined with them in praising the Lord.