This is not a definitive answer as I own a Creative!
But - all legal downloads must surely count for the charts - do they sample a number like they used to do with the record charts or do they get figures from them all?????
Secondly : you must be able to buy from iTunes but don't they come in some funny apple format. And aren't your Sony tracks in some funny Sony format? You would need to be able to convert from one to the other. Have a browse around on this site - in Technology and Music - I know that other people have asked questions about converting.
You could try the search facility - doesn't always work.
Hopefully some one who knows more about this than me will help you out. Good luck.