Nathan, I poked around in the code.
It seems to use some standard Gmod locations, with some fancy additional blacklist, combined with ULX playsound.
Playsound presumes all sounds are stored in /sound (literal server, or virtual addon).
So, anything in addons/youraddon/sound/yoursounds/*.* that are compatible with Source should work.
Remember though, they'd have to be on the server and the client for sound to play.