If you are here, it probably means the tutorial on my The Last of Us Infected Overhaul mod page to install zombie sounds didn't answer all your questions. You're probably wondering "Why do I need to manually install workshop files ?". The fault here is to fmod events which handle some type of sounds that activate depending on various conditions, for example:

• A zombie is passive, so it will play passive sounds
  
• A zombie is attacking, so it will play attacking sounds

All these are handled via something called fmod from my understanding and it gets the sounds from a .bank file which you can't automatically download via the Workshop because it doesn't overwrite the base game files (for reasons smh). But we need to change this file to change the zombies sounds and thus that's why we need to go through manual installations for a better quality mod and experience.

Now, let's all say "♥♥♥♥ you fmod" :D

PS: while playing sounds for zombies depending on their condition can be managed via Lua, I'm just going to give you a small thought experiment:
What happens when you have to manage thousands of zombies at once to make sure they play the correct sound by checking their position from the player, their actions, check what this action is and make sure the zombie is supposed to play that sounds ? It's not great for your game's performances man ...

It could be doable by just having way less possible sounds for the infected but tbf I prefer the quality of the current sounds.