I firmly believe that a combination of higher CD prices for a poorer product (subpar music and an increase in unrippable discs) and the explosion in other forms of electronic entertainment is at the root of the problem of CD sales, along with the big labels antiquated notions of distribution and the ownership of intellectual property.
First off, when I was a kid, we had 2-3 channels, off the air. Very little to watch and almost everyone saw all the same stuff. Now most people have access to 50 to 250 channels, along with pay per view movies and sporting events.
Next just look at videogames. A couple of years back, the gaming industry actually raked in more money than the movie industry!

Videogames meant "Pong" when I was very young, and I guess I was in the first generation that grew up in video arcades. Nowadays, I know quite a few people who have more games than CDs.
Computers is another area that can rapidly expand to fill any time you have available; just look at this forum!

I will have to sheepishly admit that there are days where I spend more time here talking about gear and music than I do actually listening to it!

And I suspect I'm not unique in this regard.
Lastly, just look at the explosion of HT in recent years. DVD has been the most successful electronics product in history, and HT is a huge business. In my own case, I'd really gotten stuck in a rut, audiowise, before the DVD came along. It seemed like technologically nothing had changed much since the CD debutted, and once my beloved 80's metal bands fell off the map and the grunge movement fizzled, I fell into sort of a stupor where nothing out there musically interested me anymore (save classical). I hadn't changed a single component in my audio system for about five years at that point. But DVD changed all that, and lit a fire to upgrade my gear that spilled over into audio, really getting me excited about gear again.
Nowadays I have about 350 DVDs, and while I'm more on a music kick the last couple months (due mostly to DVD-A & SACD, along with purchasing Nero & Music Match Jukebox), there are times when I spend more time watching DVDs than I do listening to music.
Lastly, I think the Big Five are shooting themselves in the foot. If I have a choice between spending $14 on a big blockbuster movie that I know cost $125 million dollars to produce, how can $16 be justifiable for a CD that cost at most $300,000 to produce? Sure, there's publicity, etc, but so is there for advertising a big movie. And on top of that, the studios seem to be laying awake nights wringing there hands in fear that I just might make a copy of a disc for the car instead of buying 2 copies! Huh?

Earth to record execs: get a clue.
And just why can't those guys figure out the internet could actually help them sell music? I buy virtually all my gear and maybe 50% of my music online, but I can find almost no downloadable full res content to buy.
Anyway, I know this is becoming a rant, but I do think a lot of factors are at work in the decline of CD sales. And tho they can file lawsuits till the proverbial cows come home, I think the Big Five best pull their heads out their asses or get used to their new place in the world.