Albert Barnes Commentary 1 Chronicles 15:21

"and Mattithiah, and Eliphelehu, and Mikneiah, and Obed-edom, and Jeiel, and Azaziah, with harps set to the Sheminith, to lead." — 1 Chronicles 15:21 (ASV)

Harps on the Sheminith - The word “Sheminith” properly means “the eighth” and has been compared to the modern musical term “octave.” Furthermore, “Sheminith” and “Alamoth” are regarded as contrasting terms, and the harps of Mattithiah and his companions are thought to have been pitched an octave below the psalteries of Zechariah and his brothers.

The word translated “to excel” is understood to mean “to lead,” which would identify Mattithiah and his companions as the leaders of the singers.