Why is that? It's really extreme. A note sounds so much louder than the one just half an octave below it that it's impossible to write anything with a moderate pitch range. I learned from the lmms wiki how to adjust the "base note", and it seems that the volume is determined by how far above the base note it is. Is there any way I can make the volume constant?
Volume is constant. It's your hearing that isn't.
Read up: http://en.wikipedia.org/wiki/Psychoacoustics