Regarding whether the addon is safe:
There is no safety guarantee that can be given, that’s why the addon policy exists with warning icons in the library before installing. You can read it here #1259477034959114341 or on the website.
I personally looked at the source code and I saw nothing wrong with its implementation when I added it.
It is closed source as requested by ArenaNet for all addons that reverse engineer the game.
Regarding your system audio:
This does sound unrelated since the addon does not interact with your system audio as far as I know, but @dreamy hornet should be able to maybe answer this.