As it turns out, television stations aren't actually turning up the volume when the commercials air. Because FCC regulations cap the maximum strength of the audio signals that broadcasters are allowed send out, the loudest TV commercial cannot legally be any louder than the loudest part of any actual program. But television programs contain a constantly-changing mix of audio levels for dramatic effect.
Most advertisers, however, just want to grab viewers' attention for 30 seconds. To do that, the audio tracks of commercials are intentionally mixed so that every moment is as loud as possible within legal limits. “Nothing is allowed to be subtle,” says Brian Dooley, Editor-At-Large for CNET.com. “Everything is loud – the voices, the music and the sound effects.”
Spencer Critchley, writing in Digital Audio, explained it as follows, “The peak levels of commercials are no higher than the peak levels of program content. But the average level is way, way higher, and that’s the level your ears care about."