Tones (and audiobooks) are a one-time only download from the store. Do you have copies of them in the Tones section of your computer's iTunes library (or on your backup of your downloads/library) so that you can sync them back ?
If not, and if they are still in the store and you haven't moved countries since buying them, you could try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/
If they do grant you a redownload you can copy them over to your computer's iTunes library via File > Devices > Transfer Purchases so that you can take a backup copy of them. (Apps, music etc will only be redownloadable as long as the rights-holder doesn't remove them from the store, if not already doing so you should also be taking backup copies of them.)