John Gill 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)

And Mattithiah, and Elipheleh, and Mikneiah, and Obededom,
and Jeiel, and Azaziah
These six were appointed to sound:

with harps on the Sheminith to excel ;
which Sheminith some take to be an harp with eight strings, or the beginning of a song, or a musical tone, the bass; see the title of (Psalms 6:1) .