I have the same exact issue with my '11 regal as well. I have the T06, but I have also reproduced the issue in other regals with the HK sound system. From what I've read on forums it seems to be something people notice but don't think much of or maybe they don't realize that the sound fluctuation is an actual problem. At first the techs at the dealership couldn't reproduce the problem and of course they had never heard of this issue before, but I was able to narrow down the sound 'fluctuation' issue to certain points in songs. For instance, if you are listening to a song on your iPod or a CD and you hear when the sound changes, you can actually rewind and reproduce the change at the exact same part of the song every time. After hearing this, the tech at the dealership thinks its some sort of calibration issue. I called Buick customer assistance and I should hear back from the regional service manager in the next day or so to find out what the next step is to get this resolved. I'll post any updates as soon as possible.