Sales of CDs and legal downloads.
The argument I've heard made, which goes back to the days of the compact cassette, is that ultimately, the more copies (legal or illegal) that there are around, the more actual sales are made, because many people will like an album they heard from a friend's pirated copy and go buy it.
The same argument was made during the Napster debate, and I questioned then, that if the music industry actually ends up profiting from unauthorized copying, why are they spending so much on lawyers to shut Napster down? (If the profit ultimately came from concerts, they still wouldn't want to shut down a free PR machine.)