In order for the Schumann frequencies to change there would have to be a drastic and permanent change to ionosphere or the conductivity of the earth's surface. As you've mentioned there are small changes and spikes all the time due to solar wind input's effects on the ionosphere.
the correlation between Schumann resonance harmonics and human brain waves is interesting but no link has been proven to my knowledge. It is interesting in the same way as the correlation between the lunar cycle and the human menstrual cycle.
One thing of note;
Too many of these people confuse sound waves (acoustic) and radio waves (electromagnetic).
They are NOT the same.