I was looking on the internet one day, and I found some hilarious mods where people would take Source Engine games, and replace all the sounds with their voice. I considered doing this myself, but I was wondering: How do I replace the old sounds with my sounds (while keeping the old ones backed up safely somewhere)?
Since the migration to steampipe, all replacement files are contained within your /custom/ folder. This applies not only to model/texture replacements, but sound replacements as well.
If you want to replace a sound, first locate it in within the applicable .gcf file, then emulate its directory using folders in your /custom/ folder. As long as the sound file in question matches the name and directory as what's contained in the .gcf file, the sound is replaced in-game.
Note that the original files are still safe and sound in your .gcf files, its just the game checks your custom folder for any files that can be overwritten and uses those first.