Theory...If the zombies are still alive, then do they shriek in an effort to save your life when they are near? In other words, can they detect that their bodies are moving towards something, and hear someone is near, so they start making all the noise that they can to alert the unfortunate person to the zombie's presence? Just a thought that I got during a playthrough of HL2...
Yeah, I believe that's true but I also believe that Headcrabs want them to have some control over their voice to some degree, Which makes their concern for others a heads-up alert to the other headcrab-zombies nearby to wake up.
Holy crap thats dark
That's actually a nice theory. My personal one is that it's more a call for help (probably to be killed) once they see that they are moving and they hear someone, but I kinda like yours more.
I think that it is also a call for help, but also a valiant effort to save a comrades life when there is no hope for personal survival.