My own anecdotal experience as someone who likes a lot of current music.
I hear a song premiered on the radio during one of the evening shows that promote new music (Zane Lowe mostly). I get really excited over this great song I just heard. I go home, find it on youtube or soundcloud (usually posted by the artist or label) and listen the hell out of it. Post about it on twitter and facebook and so on.
However, this song is a good month or two months away from actually being released. By the time it's actually released, I've already heard it many times, and I've already moved on to the next new exciting song. My urge to purchase song 1 is significantly reduced.
I try to override this for artists I really care about and make sure I purchase the song, but I'm not excited about buying it.
The massive disparity between a song being released to the world and the song actually being available to buy is probably responsible for quite a lot of lost sales.