Now before we get started I know there are scripts for this but they don't disable ALL the voices such as cinematic scripted events and some won't disable the l4d1 voices.

REMINDER!!!! only do this if you are experienced at the game and do not rely heavily on vocal cues!

Go to your C: drive and go through your program files(or program files(x86)) go to steam/steamapps/common/left4dead2 and then go through this directory Left4dead2/sound/player/survivor/ and you will see a file named "voice" rename that file to voice1 and you have now completed step 1

Now go back to the original l4d2 folder in the common folder and you should see the rest of the downloadable content folders. The next directory you have to follow is Left4dead2_dlc1/sound/player/survivor/ and repeat the same steps of renaming the voice file to "voice1". And now repeat this for the directories "Left4dead2_dlc2/sound/player/survivor/" and "Left4dead2_dlc3/sound/player/survivor/" and now you should have no survivor voices at all when playing the game.

Just follow those same exact steps as listed above except rename the files named "voice1" to just "voice" and you have now returned their ability to speak.