As for the voice acting, in any case it will be known, but *do not care* applies here. For example, I used my sister's voice several times, she is not into gaming, she is not into forums, is not not into acting - any of this kind, so by not mentioning her I would not hurt the author of the voice, but of course I did just in case someone wondered.
When author is unkown it is for other resources, for example, models. There are models that were made by Blizzard, but someone just ripped it, and ripper is anonymous. So he did not make the model, he is not the Author, but he uploaded it when he ripped from WoW let's say. So here is another issue, not really an author and stays anonymous. But in this case just providing a link makes sense, as JnM said.
In case if someone wants not to be credited. I myself do a lot of stuff, I do editing, adjust some resources, work with sounds and with models and their skins, I also rip stuff from WoW myself, but if I do so, I do not credit myself, I just credit those people whose resources I used. Or should I credit myself in every step once new resource is born?
EDIT: Just a random shot of current terrain, still in development.