OMG! So glad I found this thread. I got a used 2015 Soul EV a month ago and love it, my only major complaint being the high-frequency ringing noise.
It have analyzed the different variables, and I believe it is coming from the instrument cluster. Whenever the center display screen on the instrument cluster comes on, the sound begins, and the moment it turns off, it ceases.
I am very sensitive to these noises, whether it is from a battery charger, an old TV etc. It is interesting to not that not everyone can hear this sound, both my parents have said they can't hear the sound. And it is distinct from the electric motor whine and low speed artificial warning sound. Unfortunately I can't imagine there is a way to shut off the noise, unless one can cut power to the instrument cluster display (which would not be ideal).
We recently rented a Kia Optima and I noticed that it had a very similar, almost worse, sound emitting from the instrument cluster display, so it is an issue with not just the Soul display.
I love my Soul EV dearly, but I would probably not be able to keep this car for many years given this painful noise (for some reason it is really unbearable for me, I play music often to semi-mask it). (Fortunately we are planning on not keeping it long for unrelated reasons).
Hopefully in future models Kia will use a different instrument cluster!