Here, however, it talks about singing the songs to each other, not just speaking them. But the basic thrust is to have Christian music constantly on your mind and your heart - as a way of continual worship. A lot depends on what these words actually mean. The slow, harp implications came later, and have no bearing on the original meaning of the word. I sure hope so - otherwise there is no joy in worship.
Nobody knows what it means.
Even if we don't have the musical score, we can make some guesses about the general style of the music based on those lyrics. Harmony, on the other hand, is for the soul. As young children in grade school we are often taught that major chords are happy, and minor chords are sad. If I play a whole series of minor chords on the piano, you will soon be very weighted down and sorrowful. The minor chords depict sadness.