I’ve put up a summary of the literature on the impact of P2P on music sales that has emerged over the years. You can read the full text here but these are the basic conclusions:
An explosion in research (mainly dependent on access to proprietary data) as a result of public interest in these issues means that we are now in a position to provide answers with some degree of certainty. The basic result is that online illegal file-sharing does have a negative impact on traditional sales. The size of this effect is debated, and ranges from 0 to 100% of the sales decline in recent years, but a figure of between 20 and 40% would be a reasonable consensus value (i.e. that file-sharing accounted for 20-40% of the decline in sales not a 20-40% decline in sales).
Beyond this basic result several other very interesting facts have emerged. First is the differential impact of file-sharing on an artist depending on their existing popularity. According to Blackburn who investigates this issue the ‘bottom’ 3/4 of artists sell more as a consequence of file-sharing while the top 1/4 sell less. Second is the first tentative estimates (by Waldfogel and Rob) of the welfare consequences of file-sharing. Waldfogel and Rob’s dramatic result is that file-sharing on average yields a gain to society three times the loss to the music industry in lost sales. While, as they emphasize, this result is preliminary and based on limited data it indicates the urgent need for more research on this issue as well as the possibility to have a win-win situation in which both creators and the public get a better deal via a change to alternative compensation system such as a levy.