Woah, you are selling it on a shitload of different places! Do you manage each one individually or is it all taken care of by some form of system for distribution? I hadn't heard of half of those sites...
I will check it out then! As for iTunes, it may be a popular product, but I'm a fan of browser-based applications when they're available; not so much of Apple particularly because they've been forcing iTunes on anyone who wanted Quicktime a few years ago. Sour marketing tactics IMHO, and I'm not one to forget. :)
Cyberdevil
Hmm, have you thought about selling through a site that lets you preview tracks? Like reverbnation, bandcamp, cdbaby?
DJVI
Actually iTunes let's you preview tracks if you have iTunes downloaded on your computer. I also uploaded my song on other sites where you can buy it but iTunes is most popular site so I just mentioned it :p
Here are the list of other sites where you can buy my song. Note that the song may not be appeared on the sites yet because I just released it:
iTunes, Amazon, Google Play, Rdio, Deezer Xbox Music, Rhapsody, eMusic, Simfy,
Muve Music, MySpace Music, iHeartRadio, Nokia, MediaNet, VerveLife, Wimp, Sony Music Unlimited,
Gracenote, Shazam, 7Digital and Juke