I have had the same problem and it is separate for the AP-varying volume to speed function. It is a periodic and unpredictable "muffling" of the sound; almost as if a connection with all of the speakers is bad. I have learned to live with it by changing stations back and forth for a second and it goes back to normal.

Frankly, Porsche stereos have always been poor and I tend to listen to the engine and exhaust more than the stereo anyway.