BusinessWeek have a good article on why CD sales have dwindled away; the elastic spending patterns of the modern consumer.
DVD sales have grown, Internet services, on-demand TV have all grown at the cost of CD sales, not P2P and mp3 piracy.
CD sales or the significant portion of traditional buyers are not your P2P users. I accept a very small percentage of sales will have been impacted but no way all of the slump can be accredited to piracy. [via]