From The Seattle Times:
How much louder are recordings now? Engineer Charles Dye, co-founder of Turn Me Up with artist John Ralston, estimates that records today are 6 to 8 decibels louder than they were 15 years ago, the equivalent of about a quarter turn of a volume knob. Katz said some music that is heavily compressed has gone up almost 20 decibels in 20 years.
Advances in recording technology have allowed sound engineers and producers to raise recordings' overall volume by compressing the dynamics of the audio during the mixing process. The compression technique removes the peaks and valleys, making all parts of a song equally loud.
It's something the average person might not even notice, unless he/she listened to a recording from 1992 next to one made in 2007. People are used to being bombarded by sound, from movies and blaring TV commercials to their iPods cranked up to drown out background noise on the bus or street.
Compression has been used for years in television commercials — which are often louder than the program they are sponsoring — to catch people's attention with the loudness, sound engineers say. Now that songs are frequently bought as singles or heard on commercials rather than as part of an album, artists and labels feel the same pressure to grab listeners in a few seconds.