Hi! You already can disable sound - just open addon popup (by clicking on icon) and uncheck "Play copy sound".

Hi! I updated description. For more info you can read https://support.mozilla.org/en-US/questions/1299613#answer-1341683
Also, "Home page" leads to github repo, so you can read source code. Or download extension file (.xpi) and unpack it and read code same way.