I have no idea how to describe "Soul". I think I can perceive its presence by its effect. I would suggest that the "striving" for complete technical perfection is detrimental to acheiving "soul":
To illustrate- Recently my Granddaughter played in a High School orchestra concert. The usual fare; Mozart, Beethoven, Dvorak (sp?) all done with an almost superhuman expenditure of sheer effort - to make it perfect. And sadly, lacking "soul".
The concert concluded with "All That Jazz" from "Chicago". Brilliantly done ... a piece that most of the musicians could find that they easily related to ... most had seen the motion picture - it was "alive" and current and meaningful to them. Wonderfully done - with ELAN!! - Joie de vivre ... And 'way entertaining to experience from the audience!!
I talked to the Music Teacher / Director afterwards - to compliment her on her and the Orchestra's performance... She, overly modestly said, "There were mistakes..".
There might well have been. So what? The 'freedom', the spontaneity, the sheer pleasure of performing - all of that, and I'm sure much more - SHONE through.
"Perfect" is nice ... but "soul" - "Life" - Love of the Art" ... is so much more - profoundly more - important.