Usually the broadcasters control the volume, but if it's only happening when the HT is turned on, it may be how the HT system is handling the signal.
I'm not sure what you mean by the volume drops, as opposed to the volume "is good". Is it that the volume isn't as constant, or that audio is generally quiet on the HT, or that it fluctuates often?
Have motorola dvr(cable service)-samsung led tv un55b7100 and samsung home theater system ht-bd3252. When I have the tv on the volume is good. When I turn on the theater system the volume on several channels drops to the point I have to adjust the volume every time I change channels. Have HDMI cable connectd from dvr to tv and HDMI cable connected from home theater to tv. Also, have optical cable connected from dvr to home theater. What causes the volume to drop?