So, CC-BY-SA content can't be included in anything licensed under the GFDL, and GFDL content can't be included in anything licensed under CC-BY-SA (including Wikipedia).  I had been assuming that since Wikipedia was previously under GFDL and switched to CC-BY-SA, the latter was backward-compatible with the former.  Apparently that was not the case, but rather some kind of special temporary exemption was created—I'm not at all clear on how that would be possible, but then I'm not a lawyer.  ...In fact, I now see under my edit box that it says "you irrevocably agree to release your contribution under the CC-BY-SA 3.0 License and the GFDL [emphasis added]."  I'm not even sure what kind of exemption could, even in principle, make it possible to license content under two mutually exclusive licenses simultaneously.  
...In any case,  I doubt that the HydrogenAudio folks would be interested in suing the Wikimedia Foundation, but apparently if they wanted Wikipedia to be able to reuse their content they'd have to change their license.  Since they're a wiki too, though, I'm not sure how they could do that, lacking whatever special mojo Wikipedia used in 2009.  Lucky for me I don't care enough to waste any more time on this (but I kind of wish someone would fix it).