An interesting explanation on why sound is getting louder at:
The reason we are in this mess is basically radio ... Every record executive wants their mix to sound "hot" on the radio... they want it to sound like it has more "punch" or "impact" than the next guys - so that people will buy the record. So, they tell that to the mastering engineers...and what do the engineers do? They use dynamics processing (compression, limiting, etcetera) and squeeze everything up into the top 1 or 2 dB. this makes the whole thing louder... "hotter" - at the same time it ruins the musical feeling.