A Brief Overview:
The Loudness War (or Loudness Race) is a phenomenom that originally began in the 80's, before gaining a serious foothold in the 90's when CDs became the primarily medium for listening to music. It refers to the practice of using compression to increase the "loudness" of quieter sounds in a recording, which in turn increases the overall volume, to sometimes absurd degrees. While this increases the overall kick of the song, it greatly lowers the quality of the individual pieces of the song, which in turn lowers overall song quality.
I'm glad you asked. If the above paragraph didn't leave much of an impression on you, perhaps this visual representation (courtesy of Wikipedia) will. This imagine shows the compression changes on a single remastered song over a three decade period.
Did you notice the subtle jump from 1993 to 2000? Imagine what that looks like now.
At the levels of compression that are common in the music industry today, there are two problems that arise. The first of these is distortion. When volume passes a certain threshhold the sound becomes less clear, fuzzier one might say. Sometimes this is done on purpose by the artists recording the song. However, the Loudness War can further distort these sounds beyond what the artist intended, and can distort parts originally intended to be clear. Lowering audio quality and taking away some control from the artist over what their music sounds like.
The second of these problems, as briefly was touched on above, is the fact that the "kick" of some instruments is lowered when compared to the "kick" of other struments. Thus those instruments no longer stand out, and provide a "kick of their own when they're meant to. This video by sound engineer Matt Mayfield demonstrates this problem beautifully:
However, let's say you don't care about sound quality, or maybe you even like the music to be louder, think of it from a moral standpoint. Does the industry have a right to control the volume of the music you're listening to? Arguably, any CD you've bought over the last 20 years (Especially re-releases of older albums) has been a victim of the Loudness War. The industry has effectively turned up your volume knob without your consent. If a track starts out low you can always crank up the volume, but if it starts out loud can you always turn it down? A louder sound at a lower volume loses its kick, and any benefit from the added compression is lost, while all the downsides remain. To quote Mark Mayfield; "Wimpy loud sound".
The Good News:
There isn't any.
Just kidding. The good news is that the modern era is moving away from CDs and heading towards a more digital form of music distribution. Which, thankfully, the Loudness Race has more or less left untouched. For example, here is a compasion between the CD and digital (via Guitar Hero) versions of the Metallica song "My Apocalypse":
The bottom (digital) version of the song is much less compressed, and as a result has a much more dynamic sound. Preserving audio quality and giving full control of the volume knob back to the listener.
The Loudness War (Loudness Race) is a practice that few people are aware of, yet effects almost all music lovers. As the industry continually increases the volume of released CDs, the quality drops accordingly. The practice takes away control of the volume knob from the listeners, and can even distort the sound of a recording far beyond what the artist intended. Thankfully though, after twenty years of contunually rising volume, the practice may finally end as the new age of digital distribution takes over.