Now go to file>open and go to your cstrike folder:
Lets say you want to change what a hostage says when you use him. 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 take note of the name and location.
Now maybe you want the noise for this particular weapon to be you saying no. Well just get audacity and press the big red record button:
Record yourself saying no into your mic then go to file>save as and export it as wav with the same name as the file you found.
Now go to this path C:\Program Files\Steam\steamapps\common\counter-strike source\cstrike\custom and right click create a new folder called "my_custom_sounds", after that, keep adding folders corresponding to the location of the file you found.
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.
PLEASE READ IF YOU'RE EXPERIENCING NO SOUND INGAME!!
I have spent days trying to get sounds I RECORDED of myself to work in CSS. The problem is the FREQUENCY of the sound clip, yes it must be a .wav, must replace the old file, and it MUST BE 22050 Hz! I think certian .mp3's can be the default 44100 Hz but 22050 Hz is the solution I've been looking for. I discovered this from analyzing the original .wav files using GCFScape.
Audacity can easily change the Hz of a sound file. So if you're experiencing the same problem I was, just import your sounds into Audacity and change the Project rate to 22050.