Source Engine - How to replace sound files?

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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)?
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How do I import custom sounds into a hammer?
Locate the Steam Apps folder, open it and locate the Common folder, open it. Locate the game folder and the soundtrack will be within it. Locate the game folder and the soundtrack will be within it.Making Sound Mods for Source Engine
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Tim Samuel, Marc Schulte, Anete Lusina, ThisIsEngineering