It seems they have pulled them for security reasons and will not post them again til 19 May. For Prelude owners you can request a secure download link.
"Due to security concerns we have temporarily disabled downloads of the X-Fi Prelude driver. Drivers are expected to return to this page by May 19. Please accept our apologies for the inconvenience.Current X-Fi Prelude owners may request a secure download link via the
Auzentech Help Desk."