David and the captains of the host--that is, the princes (Ch1 23:2; Ch1 24:6). It is probable that the king was attended on the occasion of arranging the singers by the same parties that are mentioned as having assisted him in regulating the order of the priests and Levites.
which prophesied--that is, in this connection, played with instruments. This metaphorical application of the term "prophecy" most probably originated in the practice of the prophets, who endeavored to rouse their prophetic spirit by the animating influence of music (see on Kg2 3:15). It is said that Asaph did this "according to David's order," because by royal appointment he officiated in the tabernacle on Zion (Ch1 16:37-41), while other leaders of the sacred music were stationed at Gibeon.
to lift up the horn--that is, to blow loudly in the worship of God; or perhaps it means nothing more than that he presided over the wind instruments, as Jeduthun over the harp. Heman had been appointed at first to serve at Gibeon (Ch1 16:41). But his destination seems to have been changed at a subsequent period.
God gave to Heman fourteen sons and three daughters--The daughters are mentioned, solely because from their musical taste and talents they formed part of the choir (Psa 68:25).
they cast lots, ward against ward--"Ward" is an old English word for "division" or "company." The lot was cast to determine the precedence of the classes or divisions over which the musical leaders presided; and, in order to secure an impartial arrangement of their order, the master and his assistants, the teacher and his scholars, in each class or company took part in this solemn casting of lots. In the first catalogue given in this chapter the courses are classed according to their employment as musicians. In the second, they are arranged in the order of their service.