The sounds used by the addon are either included in the addons pbo or are in a separate pbo.
To change the sounds the addon uses you will need to DePBO the addon pbo with a DePBO tool such as Amalfis Decrypter 1.5......available from the Editors Depot.
You will then need to find the config.cpp that will be in the resulting folder.
Pop it into Notepad and find the parts where the addon files are defined.
Just change the paths of the files to the paths to your new sound files, which should be placed in the same folder.
If you are clever about it you can even just rename your sound files to the same names that the addon already uses, in which case you won't need to edit the config.cpp, just change the sound files for your ones.
Once you have finished you will need to RePBO the files up again, just use StuffPBO....sorry I don't have a location for that one.....or MakePBO, which I think is in the Editors Depot as well.
Planck