I think the idea of the 'soul' is a bit dangerous. In Buddhism, and in non-religious moral systems, the important consideration is the ability to suffer. If a creature (or even a robot) can suffer, then we have to consider that suffering in our moral choices. If it can't suffer then how it is treated is of secondary importance to a living suffering human. But to believers, a soul is independent of the ability to suffer, and moral obligation is to souls only. So something which can not feel or think can have a soul (a blastocyst say) while something that can feel and think has no soul (a chimpanzee).
I think the danger of this approach is obvious. Colonists who wanted to enslave native people argued that they had no souls, and thus could be killed and abused. I have been on mailing lists where Christians argued that clones have no souls. One day there may be appreciable numbers of human clones in the world. Are they to be abused? What if we meet aliens? What if we develop artificial intelligences? Will none of them have souls? Who will decide?
A radical proposal - I think we need to get rid of the idea of the soul. If a thing suffers, then it matters. If it suffers in a complex way, like a person, then it is a person. Oh - and as a minor point, I think this was one of my big problems with Buffy.