Look for a setting for Normalization. It sounds like something is trying to level out the sound to an average, that is normally done to source that has quiet and then loud passages, like an action movie. You have soft talking, then BAMMM BOOOMMMM something blows up and you jump out of your seat and wake up 3 houses near you and your dog's head explodes.
Normalization tries to keep everything around a nice average. But it maybe in your case, whatever is doing that is bringing up the sound level of the low passages, then is not fast to adjust to the lound ones, and then quickly adjusts it down. Just a theory but seems like this to me.http://en.wikipedia.org/wiki/Audio_normalization