Depending upon what country that you are in then you might be able to re-download some/all of your iTunes music purchases via the Purchased tab in the iTunes store app on your iPhone, or 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.
A device's backup doesn't contain any iTunes downloads, ideally you should be taking and maintaining your own backup of all of your downloads, as there is no guarantee that any item will remain in the store - content providers occasionally remove their items.