Compelling interview on the Dolby site (by way of Music of Sound) with sound editor Walter Murch on, among other things, why movie previews are ruining the sound for the film you actually paid to see:
… many theatres are now automated—in fact almost all of them are automated—and will run trailers in advance of the film itself. Trailers are notoriously mixed very loud. All of the dialogue and sound effects are pushed to the maximum in order to increase their penetration. It’s like commercials on television that are almost played more loud than the program itself. After all, it’s human nature to shout when you’re trying to sell something. The problem is that the tail wags the dog in the sense that the theatre-owners will then set the level of the reproducing amplifier such that those trailers will not blast audiences out of the theatre. Filmmakers then go to the theatre and find that their films are being played too low. So, in self-defence, when they go into the mix, they boost the film up to try to compete with the trailers. The result is that if the film is played at the so-called correct level, it will appear to be too loud because they’re trying to compensate for the fact that frequently it’s not played at the correct level!