What I've noticed to be true is that the NAVI will cut the volume down on the audio in the front speakers (all four of them) and use that to let the NAVI speak. The rear speakers are still on, full blast. This is just a guess, because I've noticed it too, but depending on the frequency range of a CD track or "normalization" volume of an mp3, it sounds too low, too high, or just right. I think thats more of an ear thing than a NAV thing, but thats just my educated guess from what I've experienced.