You Have to Feel the Music
One of the first pieces of advice that was given to me when I started to mix music was that you have to know what you want the music to sound like in order to be able to mix the music. I think that the advice is good, but there is a bit more to it than just what the music should sound like.
What is soul
The real objective of music or any artform is to transmit emotion or feeling. The word soul as applied to music really means that the emotion can be felt. An artist with soul can connect with his audience and transmit the emotion which is the art.
Feeling of various genres
There is a feel that is associated with the various styles of music. In order for the music to sound 'right' the engineer needs to reinforce the feel. Much very emotional music is driven by the rhythm section. If these are light in the mix the music will not sound or feel right. I once heard a local band in a major mid-western city cover a reggae tune. I knew it was reggae only because I recognized the words. There was no reggae feel and the song did not have the impact that it should have had.
Mixing Music with Soul
Part of the art of mixing music is to maintain the proper feel so that the soul in the music is transferred to the listener. With some experience an engineer should be able to pick up on the feel of the song and enhance the mix and the experience with the proper balance. The engineer should be able to hear what needs to be emphasized at any given time and make the adjustments that enhance the feel of the music. A good artist puts soul into the performance. It is the job of the engineer mixing the music to insure that the soul that the band puts into the music is transmitted to the listeners.