Except in the cases that they have lossless copies from the artist, it's all transcoded crap, mostly from crappy MP3s.
Wikipedia's also a big user, and their stuff is almost exclusively transcoded from other lossy codecs, because they care more about their self-righteousness than worldly concerns like passable quality, much less the ability for any of their users to decode it.
Anything I've looked at on Archive.org (mostly video but some audio) has the original uploaded file plus trascodes into Ogg Theora and H.264. So even if it is "transcoded crap", you're given the option of the original. Which makes sense when you've got "archive" in your name.
Wikipedia may only serve free formats, but they'll make use of stuff from Archive.org so you can click through to the original if it's there.
