It's based on a well-founded principle that people pay attention to a message if it is loud - sometimes the louder the better. Army Sergeant-Majors don't yell to hear the sound of their own voices - it's to obtain an unconditional response to an order. If you attend a wedding - no-one dances to 'background music', but when the music is turned up, you can get people to move onto the dance floor. A similar, but slightly more subtle variation is used in making adverts louder than programmes, so the message can be absorbed, even on a subliminal level.