When a voicebank is badly recorded, there are a few things you can do to fix it.

  1. Add flags in order to improve the voice and add desirable/good-sounding effects. A quick fix for some samples that just don't sound quite right.
  2. If the voicebank is your own, rerecord.
  3. Check the oto.ini. If it is missing or badly done, try redoing it in order to fix problems.
  4. Apply a filter to all samples. Modern sound editing softwares have capabilities of batch application of filters and effects. This should remedy problems that extend to several samples, where individual editing and cleanup is considered time-consuming.