The below is based on that C-Pen Core is installed and working well.

 

  1. Use Finder to open up the hard drive.
  2. Go to Library\Input Methods where you'll find CPenCore.app. See attached video!
  3. Open its preferences and Show Package Content.
  4. There you'll find two .wav files. Delete those and you get rid of the sound.
  5. Reboot the computer