The ability to use your own sounds which persist between updates has been added in the latest version of Dropzone (v4.7.5) which is available now.
After you update, to use this for the non-Mac App Store or Setapp versions of Dropzone 4, create a Sounds folder at:
/Users/<username>/Library/Application Support/Dropzone/Sounds
Once created, put your customised aiff sound files in this folder. The two sounds currently are named TaskFinished.aiff and GridDrop.aiff
If Dropzone finds this 'Sounds' folder exists at the above path and contains either of those filenames then it will use those sound files rather that the ones included inside the Dropzone 4.app bundle Resources folder.
For the Mac App Store version of Dropzone, the folder and sounds would instead need to be located at the below path:
/Users/<username>/Library/Containers/com.aptonic.Dropzone4/Data/Library/Application Support/Dropzone/Sounds