Do you mean that music and/or music videos don't show in the Purchased links ? Whether they can be redownloaded depends upon what country that you are in, it's not currently possible in all countries.
If Music and/or Music Videos show, but not all of your past purchases, then have you changed countries since buying them (content is tied to the country as well as the account), they are still in your country's store for buying (content providers occasionally remove their items), and you've checked to see if they are hidden ?
If you aren't in a country where those types of content can be redownloaded and you haven't got them on your backup of your library and they are still in your country's store, then you could try contacting iTunes Support and see if they will grant you a reddownload : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page