So this explains it....

Why are commercials so LOUD? Compression. Advertisers use compression so that everything in a commercial is about at the same (loud) audio level, close to the maximum allowable levels. This way, you can still hear their pitch even if you get up to go the kitchen or bathroom. SCI FI does not turn up the volume on these commercials — they're just recorded and mixed that way. Most of our shows, however, are not in an audio-compressed format so they generally have a wider dynamic range than the ads. Since we can't go back and add compression to our shows, the ads tend to sound a lot louder than the programming.