Can I add custom sounds to my CRPL core if they are not included in game?
No.
However, you can combine the existing sounds by playing them simultaneously (or include some delay between the start) so you can create quite interesting effects. But you can't add completely new sounds.
Thats bad, I wanted to add laughing to core. Thanks for help.
I find that a shame as well. Would be nice to squeeze in some voice acting dialogues. :)
Although, I'm guessing they want to keep the file size as small/fixed as possible. Textures and custom images are, in a sense, a fixed size. Sound files can be minutes long..