i got the same problem no idea how to fix it though

I fixed it, you need to open arma through steam and use the arma 3 launcher to select to mods you need not some third party launcher (i was using A3 Launcher).

the problem was that A3Launcher was using a different mod, "ryan's zombies" and not "zombies and demons" by ryan probably because they are super similar with jsut a few tweaks but the result was an error message every time i was supposed to hear a zombie related sound, typically followed by a quick death.

hope using the mods section of the steam arma 3 launcher fixes your problem