This is pretty ludicrous, but it's too funny to ignore: this YouTube posters swears he hears "World of Warcraft" and "give us your money" hidden in the whispers swirling around the Spirit Healer while dead (there is one NSFW word in the video, if that rubs you the wrong way). Personally, while I do hear it, it's a little too off to be taken seriously. And of course we all know that subliminal messages don't work anyway. But still, if you're the type to go for a good conspiracy theory, it is in there -- maybe the reason Blizzard has the top MMO ever made isn't because it's a great game, but because they're transmitting messages to your brain!
The question does remain: what's really being said in those death whispers? Whatever it is, odds are that it's not in English. My guess is maybe Titan, the language of the Titans -- but that assumes that whatever's speaking to you is from the world, and there are probably more deceased spirits floating around Azeroth by now than actual living creatures on it. For all we know, it could be the Others.