Sing unto the Lord with the harp - A song or psalm accompanied by the harp. On the harp. See the notes at Isaiah 5:12.
And the voice of a psalm - The voice in singing; a musical voice. Let it not be mere instrumental music, but let that be accompanied with the voice uttering intelligible sounds or words. The only proper use of instrumental music in the worship of God is to deepen the impression which the words are adapted to make; to secure a better influence of truth on the heart.