I'm trying to replace one of the weapon sounds. I'm attempting to do this by changing the extension of one of the .gro files to .zip, replacing the .wav inside with a new one, and changing it back to .gro. But then when starting the game I get this error message:
"Cannot open file Conent/SeriousSamHD/textures/GlobalAtlas.rsc"
There doesn't really seem to be any modding guides or anything to learn from that I can find so I don't know any other way to do this.

EDIT
So.. I got it to work, but what I had to do was subscribe to a mod from the Workshop and replace the contents of its .gro file with my sounds. This is the only way I've gotten it to work without the game crashing. I don't know why.