The following does not replace the need for independent thought or research. it is there to support. And, obviously, not everything in the links below is designed specifically to answer the A2 question, but all links do contain useful information which you can use.
History of iTunes (in need of updating)
Some more on iTunes
The Impact of Online on the Music Industry (yes it says you have to pay for the whole thing but if you scroll down you'll find you get a few excerpts gratis)
A candidate from 2009's thought son Spotify (brief)
A candidate from 2011's thoughts on HMV
Obviously expecting much more detail, much more interactivity and generally more Awesome from you that the last two posts which obviously were introductory research (some good ideas there) so they're good starting points.