Ok, firsts things first you need GFCScape, you can find it here: Nemesis.thewavelength.net That's the latest version I beleive.
Now, you might have to close down Steam for a few minutes when your searching for your files you need because Steam opens up your GFC files and doesn't like other programs opening them when they're in use so close it. Now that have GFC Scape open there should be a icon right below File click it.
Get this path and follow it Computer/ C:/ Program Files/ Steam/ steamapps/ I think that's the right way I'm not 100% sure. Ok, now find what ever game you to mod. (mainly Source or Gold Engine games like TF2, HL2, HL1, HL1 or 2: Death Match, etc.). Once you find the sounds or what ever your looking for extract them, Desktop seems to be the best spot for ease of access . Close GFC Scape.
Renaming: Ok, if you have any sound files of your own rename one to replace the sound you want to change like here for example (Using zombie noises lol) Your sound ,rename it to like zombie_loop1 then replace the original sound with it. Now do that like 100 more times. So when ever your going to replace something just rename your sound the current sound you want to modfiy.
Intalling: Install loacations may vary from what game your modding. (Example for Half-Life 2: Computer/ C:/ Program Files/ Steam/ steamapps/ USER NAME/ half-life 2/ hl2/ sound/
(Example for Half-Life 2: EP1: Computer/ C:/ Program Files/ Steam/ steamapps/ USER NAME/ half-life 2 episode 1/ episodic/ sound/
(Example for Half-Life 2: EP2: Computer/ C:/ Program Files/ Steam/ steamapps/ USER NAME/ half-life 2 episode two/ ep2/ sound/
(Example for Portal: Computer/ C:/ Program Files/ Steam/ steamapps/ USER NAME/ portal/ portal/ sound/
(Example for Half-Life 1: Source or original: Computer/ C:/ Program Files/ Steam/ steamapps/ USER NAME/ half-life source/ hl1/ sound/
It's a very very long list I'm not going to go through all of them because you need to learn a bit too right? Anyway, that's the tut to modding sound files so yeah and while your here try out my Tourettes Guy Zombie Mod! I'm taking requests also might have to wait a while due to school. So enjoy modding!