It was said, that when one piece of music is often downloaded, the sales increase. I don't find anything special about it. When it's popular, more people buy it, and more people download it.
There is a possibility that somebody downloads mp3, finds it very good and decides to buy it on CD. But on the other hand, somebody who's waiting for an LP (and wants to buy it) downloads mp3 before album is released and finds it not good. Then he doesn't purchase it at all.
p2p decreases music sales, especially in developing countries. It's not only the mp3 thing. Images of almost every CD can be found on the net, as well as CD-RWs in people's houses.
Finally the most importand thing. When music companies set high prices, people steal music from p2p. There is no other way to listen to the music for many people in the world. If p2p was stoped, the sales wouldn't rise. People would steal music in other way. You can call music companies' policy stealing as well, but it's another topic.
There are two solutions. One is decreased prices. It's better to sale 1 mln copies for 5 euros, than 0.2mln for 15. I can tell you that it works, it's not just a theory. In Eastern Europe you can find cheap CDs (1/3 of average price). They always occupate first places on top sells lists. Doesn't matter if they are good or not.
Second solution is internet. Selling throughout the net, for a reasonable price, is the future market, which will wash away the present one. No matter what the music industry thinks, changes are coming.