I corrected the key signature in Global tracks, and three already-recorded vocals now sound as if I had processed them through a voice-disguising filter. I returned to the original key signature, and the problem went away. Inexplicable. Am I stuck with the wrong key signature?
That is VERY strange. Really the only way any audio can/should be affected by the Key Signature track is if they are Apple Loops.
This _shouldn't_ make a difference either way in this case, but just to rule out possible voodoo: what happens if you disable the "Follow Tempo" checkbox for these audio regions in the Region Inspector?
And again, just to rule the extremely unlikely, is Flex Time off?