It is true that it is not possible to define soul in any logical formalism , but then so is the case with the impact of percepts experience through our sensory faculties, it is not possible to precisely define them. The spiritual experience people claim they go through when they are performing some rituals(like Namaz) or when they are in the vicinity of certain places (Like Kaaba), it is not possible to validate their claim but then it is also not possible to invalidate it, when in essence that too is just a mere orchestration of specific sequence of sensory stimuli. My point is that all experiences that people claim have had an impact on their soul can be described in terms of basic sensory stimulants , their primitiveness does not constrain them for causing such sensations that incapacitate our ability to define them under a cause & effect framework. So is the case with music & soul.., no definite statement can be made about it, but discarding the impact of music on our soul is in itself making a definite statement about an incomprehensible phenomenon.
Very good.
I just have to mention that the example of namaz or seeing kaaba and its effects which depends on people's willingness to experience the effects.
A non-muslim will look at Kaaba in a different way.
Music on the other hand can have universal or near universal effect.
Other point to consider is that soul is an undefinable part of human being. Most of the people confuse this with 'feelings' and others just keep saying it is a metaphorical term. Anyhow, 'soul' and music relationship is not really proven.