I think I have the same problem but I found a fix so I'm curious...
Go to "tone" and take it from "hk surround" to "stereo"
See if the volume changes still happen.
Stereo doesn't exactly sound as good as HK Surround, but the whole point of surround is through manipulating the volume (my receiver / stereo at home does this too) and so if it's a certain song you want to hear without volume changes then do this.
Also, set all of your levels to equal at mid way (treble, mid, base all the same at the middle bar).
Whenever I have it set on "stereo" and all the equalizers level then I don't have any volume changes... can others back this up?
I think a fix for the HK Surround would be great, but I think it would be much more complicated.... they would have to redo the speaker layout for the car's programming (it's as if they put in a front, mid, and mid speaker instead of a front, mid, and rear speaker and that's why it's dropping - there's a speaker missing that's supposed to pick up that part of the "surround" and area range)