I requested something similar a while ago (adding compression to audio so the adverts wouldn't be louder than the programme). I was pointed in the direction of using ffdshow (with a compression plugin?).
Never got around to trying it as I have since discovered commercial skipping via comskip/showanalyzer
FFDShow is a directshow-filter. If you have it installed, and selected in GBPVR-config-playback-tab (ffdshow postprocessing). Then you configure ffdshow to normalize the volume, I even think it's a default setting in ffdshow. I don't know if it works though:confused: