The screenshot sound in my Ubuntu OS was so annoying to me and therefore I decided to get rid of it.
The best way was to change the file name or move the file to a different location, and I decided to move the file.
So below is the answer to your question ‘How to disable screenshot sound in Ubuntu?’
sudo mv /usr/share/sounds/freedesktop/stereo/screen-capture.oga usr/share/sounds/freedesktop/stereo/screen-capture-disabled.oga
Hope it works for you as well.