Cepstral uses installers to add the voices, so I'm quessing this is how you installed it (rather than placing it manually whereever the other voices are)
Do you have a 'Cepstal Voices' in the 'other' section of System Preferences? If you do, you could try uninstalling the voice. If that doesn't work then just talk to Cepstal, I believe they are very customer friendly and well answer emails promptly (when in doubt, go to the source.)
And of course the old fallback, repairing permissions, which can be done using disk utility in your utilities folder.