How to disable the "waterdrop sound" played when locking or unlocking the screen
You don't need to edit any sound files as the Screen Lock app has provided this as an inbuilt feature to disable/modify lock and unlock screen sounds.

Open Screen Lock app and open Settings, scroll down to the bottom and you will be able to find the option to uncheck and turn off the sound when locking and unlocking the screen using the widget or notification bar.



Screenshot - Screen Lock Settings - (Click to enlarge image)