Rashi on Amos
Who sing according to the tone of the lute—Menahem (Machbereth Menahem p. 145) classifies it with (Lev. 19:10) “And the single grapes (וּפֶרֶט) of your vineyard,” and I approve of his view, for the one who sings with a musical instrument separates the speech individually, everything according to the tone of the instrument, whether to raise [the note] or to lower [it].
Rashi on Amos
they thought their musical instruments were like [those of] David—They thought their musical instruments were like the musical instruments of your servant David, which were meant for the sake of Heaven, but these were for enjoyment.