The Eva roars are basically modified human vocalizations. Try shouting into a microphone, pitch-shift it down, and you'll probably end up with something fairly Eva-like. There is more to making an Eva roar than that, I assume, but that's the secret of NGE's sound engineers… In my opinion, the Evas' various roars, grunts, and bellows have not been even adequately replicated in any dubbing efforts (one has to wonder why the Evas need to be dubbed over in the first place…). Even Gainax's redubs for DEATH were inferior — there's just no beating the original.
thewayneiac: In the commentary for the English dub of DEATH, AWL says they had to dub the Eva cries because they were on the dialogue track instead of the sound effects track. Wow, some actual information!
Reichu: Notice here how a joint in the mandibular piece pops open as she opens her mouth as wide as it (one would hope) goes… I'm not sure why this piece of armor would need such a joint, though.