The Iron Maidens, an even more nightmarish variant of the Regenerators, sound even worse.
The sound of a chainsaw and "Te voy a matar!"? signals one hit killer elite mooks.
Another one from the 4 is the dry, shuffling sound made by the Novistadors. Particularly horrific because they're essentially invisible, half the time. Not to mention the first encounter in the sewer. A very loud SPLOSH followed by an increasingly louder series of sploshes indicating footsteps.
The chanting of the Illuminados Zealots: "Cojelo, cojelo, cojelo, morir es vivir, morir es vivir, HAHAHA!" :WHACK: