My XC90 sound system was always loud and amazing, even at a low volume setting. The output drops when the auto start/stop kicks in and you can hear the difference. This is to manage the battery drain I am sure. Recently I noticed that the sound is not as loud as before, at low volumes it is not as audible as before. Need to crank it up over a quarter of the volume setting.. what could be the reason, could the battery voltage be low?