I guess I'm a bit stupid......
I installed GCFScape, and it's a simple enough program to understand.
It opens the half-life.gcf file just like any browser, and lets me see the file I'm interested in, in my case the materials textfile. Problem is....I can add my texture to the kind of sound I want, but I CAN'T SAVE it!
The program even lets me export the file to any given location, where it can easily be edited. But then I can't import it again, hehe :
As I said earlier, it's no big deal.