Yes I know it's DoD but it works for CS:S all the same. Made it for a guy on another website.
First get GCFScape from here.
Install it and open it.
Now go to file>open and go to your steam folder:
Lets say you want to change the sound of the blade swinging for the allies. Well just go to the sound that looks like its named after what it does. You can even double click it to preview it. Once you find the sound you want right click and extract it to your desktop.
Now maybe you want the noise for this particular weapon to be you saying no. Well just go to windows sound recorder in your start menu accessories folder
Record yourself saying no into your mic then go to file>save as and save it over the file you extracted to your desktop earlier.
Now go to this path C:\Program Files\Steam\steamapps\YOURNAMEHERE\day of defeat source\dod and right click create a new folder called "sound"
After you have created that folder enter it and create another one called "weapons"
Now take the .wav file you made of yourself saying now and move it here.
I got all these paths from GCFscape in case you want to do a sound for something else so just look at the path you are extracting from in GCFscape.
Now go in-game and try it out. :D
