I downloaded a CD but some tracks are not complete. Can I download again?

Whether you can re-download them depends upon which country that you are in (music can't be re-downloaded in all countries). If you are in a country where music can be re-downloaded then try deleting the incomplete tracks from your iTunes library and redownload them via the Purchased link under Quick Links on the right-hand side of the iTunes store home page on your computer's iTunes : re-downloading.

If you aren't in a country where you can re-download music or if they re-download in the same state then try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find those songs and use the 'Report a Problem' link.

In New Zealand (from this page) you should be able to re-download music, music videos, films (though maybe not from all film studios), apps and ibooks - if that album is still in the store then you should be able to delete the incomplete tracks from your iTunes library and re-download them via the Purchased link under Quick Links on the right-hand side of the store on your computer's iTunes home page