Arguments over text are only possible because there is a record of it in the first place. We know the works of Homer because someone thought to write them down, not because they're still being performed and repeated (some date the first written records of the Iliad, for instance, to around 800 BC). Languages change, sure, but nuance and interpretation doesn't change that the text remains the text.
The real problems with oral tradition are that it's possible to alter or erase parts of the recitation over the course of a single generation, and that it's very, very, regional. The only way for oral histories to remain intact is to prevent interference, and as cultures interact with each other, that is less and less possible. The only practical way to store knowledge and spread it is by recording it somehow. If you're not really interested in spreading it beyond your community, that's fine, but its scope and impact will always remain limited to it.
I think you're arguing that it's possible for the names and stories of the Old Ones (or Elder Things, or whatever it is that's being prevented from coming down to Earth) to be passed down orally rather than writing them down to escape the Archive's notice. I'd say it's possible, but that the vast, vast amounts of time would have rendered generational recollections almost useless for it to matter in the Oblivion war. For instance, names need to be pronounced correctly, and regional dialects and accents would play merry hell with that. Even simple, monosyllabic names like "Bob" have difference pronunciations depending on where you go. I knew a guy from Kentucky who pronounced it with two syllables ("Bow-ahb").
It's perfectly possible within the Dresden Files universe, of course; someone like Bob the Skull would be perfect for recording this kind of thing, and I'm sure he'd be able to play it back as it was said to him like a recording.
The problem with the digital age, I think, is that I'm not sure whether the Archive knows about video and audio. Imagine some low-level warlock recording a summoning, posting it to YouTube, and the whole thing going viral. Warlocks going open-source would be... interesting. I know Harry says the White Council does that kind of thing intentionally, but I'd imagine the Old Ones don't care. You can't, you know, delete the Internet.
Anyway, would the Archive know about that when it was uploaded? Or would she only know about it when someone wrote about it on their Facebook page? Because there's a distinct difference, I think. I suppose, worst case, she'd know the machine code basis for the video file as it was created, and could compile it mentally, but that would be... taxing. And borderline impossible, considering that there is, like, several hundred hours of video uploaded to YouTube every minute or something.
Anyway, if I seem like a jerk, I'm sorry; I have strong opinions about writing in general. I've heard way too many people tell me that they have a great idea for a novel, and I have to keep telling them it doesn't matter if you don't write it.