If the parts if the soul are: the emotions, intellect, heart, mind, will, desires etc then “soul harmony” would be a beautiful blend of all their voices joined in one song in perfect leveled tones.
If we don’t have soul harmony, and yet it’s something so possible that we’re told to “let” it rule – then one or more of the voices is out of time or tone with the Lead or the internal choir.
Peace of Christ = Soul harmony