can you check if this is happening on your setup, start a character default no professions no skill leveled,sneak up to a zombie and whisper (shout command) more often than not the zombie does not react...i had this happen while there was a fog so i assume the fog makes zombies not hear a thing? although once i start walking normally zombie reacts,again this does not happen every time but most of the time. let me know what you find ill keep checking see if it's consistent enough to write up a detailed procedure to trigger this.