My original EPUB2 version validated perfectly -- it's already up on the iBooks store, in fact. I just opened that, ran the EPUB3 converter plugin, ran it through Pagina where I got a ZILLION errors (give or take a few million)
fixed all the
errors -- but still had those hundreds of "warnings" -- and then added in that one line of code for the audio file.
Oh, and I've been doing all this EPUB3 editing in Sigil, too -- would that be the issue here? I realize that that is (or at least was) meant for EPUB2 editing.