Charles Ellicott Commentary 2 Chronicles 29:27

"And Hezekiah commanded to offer the burnt-offering upon the altar. And when the burnt-offering began, the song of Jehovah began also, and the trumpets, together with the instruments of David king of Israel." — 2 Chronicles 29:27 (ASV)

Commanded to offer the burnt offering ... altar. —These words are repeated from 2 Chronicles 29:21, because all that comes between is descriptive of the preparations made for the due performance of the sacrifice.

When the victims had been slain, flayed, and cut up, and the altar had been sprinkled with their blood, and when the Levitical musicians had taken their places, instruments in hand, everything was ready, and the sacrifice was ordered to begin. And at the time when the burnt offering began, the song of Jehovah (that is, the chant of the Levites with its musical accompaniment) began, and the clarions; and that under the lead of the instruments of David king of Israel, that is, the harps and lyres were dominant throughout, and the trumpets subordinate to their music. Or we may translate: “And that at the side of (that is, along with) the instruments of David king of Israel.” The phrase is ‘al-yĕdê, “upon the hands.” (Compare to 1 Chronicles 25:2–3; 1 Chronicles 25:6.)

The Septuagint omits the needless “and that” (wĕ); the Syriac translates: And when the burnt offerings began to be offered, Hezekiah began to chant the praises of the Lord, as from the mouth of David king of Israel. The Vulgate also is very free.