In rock, the words get used in the same loose ways of language - the spirit of the age, the soul of the band, the heart of the matter, Doctor Love. In the tao, the differences are specific. The immortal fetus is a Soul body. Spirit work is of this earth. Body areas are described at subtler levels of mastery in terms of which part resonate with Spirit work and which with Soul. I can paste more of this literature if you like. My own assimilation is kind of in agreement with other comments here - soul is this enduring thing that is of the unity whereas spirit is something more conjured, more specific. Spirit works and plays in the name of the one Soul.