Not suprisingly , IFPI claim that piracy is the biggest threat the the music industry ever and that it is solely responsible for the entire drop in music sales. RIAA site isnt responding to me, but I suspect they make much the same claims.
http://www.ifpi.org/
These people quote Jupiter Media Metrix as saying there is no noticible impact:
http://www.rediff.com/search/2002/sep/09mp3.htm
and :
http://www.newsfactor.com/perl/story/17600.html
Are quoting JMM research indicating a sales boost due to P2P.
Back from the days of napster:
http://www.wired.com/news/business/0,1367,36961,00.html
Note the continuing claim - a substantial proportion of P2P users download music to help them decide what to buy, and a lot of people to get music that isnt available.