Charles Ellicott Commentary Deuteronomy 31:19

"Now therefore write ye this song for you, and teach thou it the children of Israel: put it in their mouths, that this song may be a witness for me against the children of Israel." — Deuteronomy 31:19 (ASV)

Put it in their mouths, that this song may be a witness. — This method of perpetuating the truth was even better adapted to the times and to the condition of the people than the delivery of a written law. It was not possible to multiply copies of the law among them to any great extent; but the rhythmical form of the song would make it easy to retain in their memories. There is reason to believe that Samuel, the first person who (as far as we know) accomplished anything significant towards establishing a system of religious education in Israel, employed the same means for this purpose: namely, psalms and spiritual songs.

The first companies of prophets were evidently singers and minstrels (see 1 Samuel 10:5–6; 1 Samuel 19:20–24); therefore, they had a remarkable influence over Saul. And if they taught the psalms to the people, as they learned them under Samuel and David—especially historical psalms, like the 78th, 105th, and 106th—a very effective means of spreading the knowledge of God in Israel was in their hands.